I'm talking about the things she has that are actually useful. The portals and the obedience necklaces which she did not produce. When Rowan asks if they can use another one she admits she doesn't understand how they work or how to reproduce them. She very much stumbled into the most useful things in her arsenal. Unless she is lying but that implies producing those necklaces is hard and she is limited in how many she can produce because otherwise they should be using that on literally every servant. The other two things are incredibly minor and honestly unnecessary. Anyone could have done it and probably done it better.
As I said, there is nothing he did that couldn't have been done by a fearsome and dumb orc. When it comes to armies its about numbers, equipment and logistics. One fighter makes little difference (majority of fighters in battle won't even see him, a soldier in an melee scrum is as blind as a bat, ask any historian) and without Rowan planning the battle he would likely have managed to get himself killed anyway. Hell Ulcro could have easily been negotiate with, the gifts for Delane where all from outside the camp. After that you can easily convince him to allow Rowan to talk to her. You don't even need to operate in the camp realistically (which likely could have been achieved regardless).
It totally does. They spend more time getting in the way of their own plans and alienating their own servants for a whim than actually moving towards their goals. All Rowan would need to do to collapse their campaign is die. You said yourself their biggest "talent" is finding people to do everything for them and I see nothing about the way they act that it isn't writer fiat that they are able to do so.
Maybe, but like I said, them finding and being able to get these talented individuals is like their necklaces. Plot convenience to make mediocre villains not fail instantly. It doesn't speak to their ability of which we have seen very little. Yeah they are powerful in a one on one fight. They'd still have lost to the first organized resistance they encountered as we've seen if Rowan fails in preparing.
I’ll admit I haven’t given the game a full playthrough in several versions, but last I saw the necklaces mentioned, Jezera made them. Last I read, the portals were hers. She specifically says in at least a couple scenes from those versions, and at least one in 3.0.5 that I can recall, that her particular talents are enchanting items and creating portals. I haven’t read the scene you’ve mentioned with Rowan, so I can’t comment on how accurate I find your interpretation of it, but as I understand it, the only thing Jezera lucked into was being born with her particular talents.
Andras could not be replaced with an orc. He’s been shown in combat with some of the strongest orcs in his army, and has humiliated them without effort. He can throw magic around that easily clears enemies from around him, which makes him not only a dangerous weapon, but also a highly visible entity on the battlefield with all the morale effects that would imply. The mere psychological impact of having a full fledged demon lord in battle is significant for both sides, and this is without factoring in that Andras, for all his arrogance and faults, is far more intelligent than any orc leader, and is therefore a more competent battlefield commander when he isn’t letting his pride run away with him.
Rowan never would have seen Ulcro without Andras bolstering his reputation. He’d most likely have been killed.
The writing has consistently pointed out that the twins are good at recognizing talent and recruiting it. Andras could have just slaughtered Draith when he was found skulking under Bloodmeen. Greyhide could have been passed over. Shaya could have been forgotten. The twins could have let their arrogance get the better of them and demanded more than the lip service they receive from Cliohna in exchange for her use of the library. Hell, even the clockmaker could have been simply tossed in with the rest of the menial slaves, but Andras recognized the man’s talent with his hands at a glance. When characters are consistently portrayed with a particular characteristic, that isn’t a plot device or writer fiat, that’s called good characterization.
It’s this talent for finding people suited to the tasks that need to be done, figuring out what those tasks are, and outlining goals that make the twins effective leaders. As I have repeatedly conceded, they couldn’t manage the minutiae or coordinate their subordinates well themselves due to their personal failings; what allows them to be effective in spite of this is the fact that they recognize those faults and recruit somebody with the talents to keep the wheels turning. No leader can be good at everything, and while Andras and Jezera would be far more effective without their tendency to get in their own way, they’ve worked around those flaws using the talents they do possess to bring everything they have together. And don’t forget, Rowan is mostly just organizing things to make their plans work; the twins are the ones who are managing the broad strokes of their plans, and so far those broad strokes have been effective. The leaders figuring out the big picture and delegating how the smaller steps will actually work is fairly typical in any hierarchy.
You also have to remember that they enjoy tormenting Rowan, so many of their excesses seen from his perspective are at least partially theatrical—you tend to see them acting less erratically when Rowan isn’t around. That’s not to say they don’t make blunders, merely that when they’re serious, they aren’t quite as flighty or fickle as they often act.