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.
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 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.)
