I do not think it is quite correct to assume the Twins would have already failed without him, I think it is a safe bet to say everything up to Raeve's keep would have gone exactly the same and it was much more about them testing Rowan as an agent rather than needing him as one. Even after that I'd say things would have gone similarly, the orcs would have been recruited, the Rastedel army heading towards Raeve's keep would have still been sniped, it would just probably not have been quite as clean. Same even for the actual conquest of Rastedel, Jezera had agents in the city long before Rowan step foot in it, without Rowan maybe they don't manipulate the coup into happening at such a perfect timing, but they would have still had the forces to conquer the city.
Being able to protect Alexia and mitigating the Twins' damage have been Rowan's personal justifications from day 1, but it only really works because of the assumption that he couldn't have really stopped any of what they have done up to this point, which I do think is true. But at some point Rowan's impact as an asset will stop being about just making things easier for the Twins and will start being about actually enabling them to achieve things they wouldn't be able to without him, and I think that starts with Rowan eliminating capable opposition. The question becomes about for how long Rowan can justify what he's doing as being the lesser evil.
Of course this goes back to the meta-level discussion that this is a videogame and we know Werden and Marianne won't be as critical as that, but narratively speaking, the idea that making sure Werden and Marianne die pathetically without being able to lift as much as a finger against the Twins was the first real thing they maybe wouldn't have been able to do without Rowan is a little interesting.
They could probably have taken Raeve's Keep on their own, but I don't know that they could have gone past that. Bear in mind that their original plan was "Ally with the Dark Elves, then march on Prothea", which... Well, if they could have pulled it off, they'd have conquered the Six Realms. It's just that they had about a 0% chance of doing that. Plus, both of the Twins were rather overconfident prior to that; Jezera thought she'd have the Dark Elves eating out of her hand, but lost her temper and killed an important negotiating partner, while Andras had dreadfully underestimated what the Prothean legions could accomplish prior to actually reading a book.
As for the resources they had at hand... They had a small band orcs that Andras kept killing for failing him, Jezera's band of eclectics, a few mages of dubious loyalty, the ruins of an evil castle, and Cla-Min. No scouts, no cubi, and no equipment. If they wanted to build a secret society, that would probably be enough to make some serious headway, but to openly conquer a kingdom? It would have been a fantasy without a hero, and a whole lot of luck.
Plus, they were pretty panicked at Astarte; Jezera's first thought was to retreat, while Andras would have brashly thrown his life away in a doomed battle. They had the tools to win without him, but not the knowledge of how - it was only by Rowan breaking down the situation that Jezera realized how powerful her contribution to the battle could be. Andras wouldn't have been interested or able to talk things out like that.
And while I say that "they had the tools", I'm not so certain of that in practice - they were able to get an Orc army because Rowan was able to settle things in an Orc way, ensuring the legitimacy of their warchief. If it needed to be left to Andras, he probably would have just killed one, and intimidated the other - leaving the followers of the losing side discontented and not
truly loyal to the chief who now appears weak in the eyes of the tribe. It would be a smaller, weaker army, when they were already outnumbered on the field.
Prior to Astarte, Rowan's contribution was really just to make the Twins a credible power at all, organizing what they already had - but Astarte is where he really started to make the Twin's dreamed conquest a reality, instead of just speeding up something that could have happened without him.
My impression is that Werden is uncompromising with the notion that peasants shouldn't challenge nobles because he prizes the maintenance of order above all else, not because he is particularly more cruel or demanding with the peasants in an individual level. This doesn't help much during peace times where the peasants are the only ones suffering and Werden needs to negotiate for the support of the nobility, but I do expect better from his leadership during war times.
Of course that might not mean much for you since you are mostly focused on meaningful lasting transformations when it comes to the results of the game, but I never really thought that far ahead when trying to come up with merits for Werden's route, just them being able to keep the kingdom from being quite as destroyed during the war is as far of a reach as I could picture.
I agree that Werden doesn't seem particularly
cruel to me; cold and callous, yes, as one can see from his conversation with Rowan as they burn the homes near the wall, but I never got the impression that he particularly enjoyed being a heel.
But his philosophy still has a disproportionate impact on the peasantry because he
can enforce it on peasants in a way that he cannot do unto his peers. If a peasant rises above his station, he can push them back into the ground, even if it's one of the Six Heroes; if a fellow noble decides to throw a feast every evening, the most he can do is give them dark looks and snide comments. Heck, such a noble might even have more soft power than him because they're networking at the capital constantly, while he's the irritation who keeps demanding difficult changes to protect the kingdom.
In another light, it's an example of how an uncompromising philosophy that doesn't consider the specifics of the problem can cause a hell of a lot of harm despite the best of intentions. Just how Werden inadvertently launched the events of the game when he ensured Rowan would be sent back to the farm with no protection, honestly.
As far as the second paragraph goes, though - you'd be right. I don't care that Werden means well, just what the outcomes are likely to be. It does raise another thought on my part, though; how our different views on the Twin's success without Rowan weigh on the value of Werden. From my perspective, Rowan could have ended this already by dying, so saving Werden is just an ill-considered half-measure on his part; a sop to his conscience because he couldn't go all the way. But if the Twins
could have taken Rastadel without him, then he did indeed have a meaningful impact on the war situation that outweighs his own contributions to the Twins cause.
(Of course, if they could have won without him, then he really didn't need to beat himself up that much after Astarte and Rastadel, but... Details. He has no way of knowing the answer either way.)
I won't breakdown the rest of your post because I agree with some of the stuff and for the ones I don't I feel discussing it by parts would have me repeating previous points, so instead I'm gonna focus on the general picture and I think that a critical source of disagreement for me is that I think your arguments place too much importance on the "top level management" and not enough importance on the "middle level management" and on the culture that dictates their actions.
What I mean by this is that, sure the Twins are only two people and they aren't chaos fanatics, so upending every form of artificial order existing in the world is not something they are really interested in. But some of their followers are, and the Twins still preach "might makes right" as the source of their legitimacy, even if they start going down a more feudal route with titles, their presented rationale is never "accept our titles because it is for the greater good" but "accept our titles because we are stronger than you and so we can command whatever we want". To me this is an important distinction in the ethical foundation of any society.
Take as an example the governing of Rastedel, even if Rowan could somehow get the Twins to make a public announcement fully endorsing his chosen governor and saying their word is law, that wouldn't stop other believers of "might makes right" to still be willing to challenge Jacques or Patricia, specially if they actually tried to reach for more humanitarian values, because they think themselves stronger than these pathetic humans and they would only take direct orders from the Twins or someone stronger than them, of course the Twins could take offense to this and slaughter that person for their impertinence, but they could also agree with their argument and patronizingly tell the human governor they are being too soft, as we have seen such situations play out in the Twins' own castle and Rowan struggling to maintain a chain of command.
And this is all assuming the issues were even being brought up to the big shots like Rowan or Jacques/Patricia, a city the size of Rastedel will have plenty of nameless administrators that will be even further removed from the Twins and have even less power and legitimacy to keep their underlings from challenging them, and this is still assuming most of the city staff won't be replaced by orcs and demons that subscribe to "might makes right" and are absolutely fine with many civical transgressions that could occur under their watch. You can argue Rastedel will eventually reach a new balance and "get used to their new neighbours", but I really don't think any new status quo will leave them much better than slaves, with constant rapes, thefts, and other forms of abuse going completely unpunished by the new authorities, with or without the Twins' involvement.
I simply do not think a society holding to principles of "might makes right" (even if said principles aren't being taken to their logical extreme) can scale to functional leadership of a city the size of Rastedel, it would require a revolution led by iconic leadership championing more communal values, which the Twins aren't because they are outwardly impulsive, selfish, and imoral fucks.
I think you're underestimating the power of cultural inertia here, though. These are almost exclusively human settlements, who have held to Solansia's order for generations, and from a historical perspective, it is far more common for conquerors to adopt the culture of the culture of their new land than the converse. If the people at the top are actively disinterested in changing that culture, I just don't see how it happens, especially in less than a generation's time.
I mean, is anyone in Rastadel particularly interested in adopting "Might Makes Right"? We could repeal the laws on murder tomorrow, but that doesn't mean we're suddenly living in The Purge - so long as humans just aren't that interested in wantonly slaughtering each other, the fact that something becomes
possible doesn't mean that it will happen. And it doesn't even look like things went that far; remember how Rowan can decide to give a village to a mercenary instead of an orc, and justify it because he's still the stronger than either of them? He still makes the rules; the place isn't in a state of anarchy during the occupation regardless of what philosophy the Twins embrace, and I strongly doubt that he's interested in having his governor displaced by force. I doubt the Twins do, either; they know all too well that it's a quick path to having a disloyal governor, considering their approval rating amongst the city. And within the city, the words of the governor are law. They
have to be, or else there's functionally no governor in a situation like this. Neither Jacques nor Patricia are interested in having their staff picked by force instead of their usual selection process.
And this is further discounting the likelihood that the Twins are willing to make special exceptions for their richest, most productive territory. Even if they would encourage a more lawless philosophy in the countryside, they're not going to appreciate the workshops that make their luxuries shutting down for stupid reasons. In fact, I wouldn't be surprised if they end up learning a new lesson from the expense of getting Rastadel back into working order. Not that they won't just push it all on to Rowan, of course, but at least remember it the next time they consider smashing something expensive.
The most I see changing in Rastadel over the course of a year is that the truly useless nobility, the ones whose
only worth is their name, losing what they have left. People like Patricia would be fine, because they have useful skills and those they've worked with know it. People like Werden would still command the loyalty of their retainers because they've earned it, while people like Delane can still charm people into listening to her. People like Raeve, who was only ever followed for his family name... Well, they'll sink to where they
should be in a world without the power of a noble title. That's the kind of thing that can change in the course of a single generation, but I doubt that it's a change many people would mourn.
(What does "Might Makes Right" even look like outside of Orcs, for that matter? They do ritualized duels, but it doesn't seem like the rest of the Chaos races necessarily interpret it the same way.)