This is my analysis on whose help Rowan could count on if he tries to challenge the Twins. Overall, I think that good, not corrupted Rowan will have it harder. Some of the NPCs could be willing to support him as a new evil overlord who replaces the Twins, but not as Rosaria's savior from the demons.
* Greyhide: He quickly becomes Rowan's friend so that would be easy in his case. (+)
* Helayna: She is also an obvious one as she is completely enfatuated with Rowan, though she could be an important ally only in a scenario when Rowan chooses to train her back into a knight (instead of making her his lover). (+)
* Cliohna: She dislikes twins, but is focused on her own goals. If Rowan pursues a dominant or at least an even relationship with her (and does not become a slave to her charms) she could help him, but only if he offers her something valuable in return. Probably can't ally with her if he is too much pro-Solansia. (-/+)
* Cla-Min: The goblin matriarch could be a difficult one, because it's clear that she values her clan's interests over anything else. She wouldn't openly betray the Twins as it would be too risky for her. She is already in a very good position and doesn't need any significant change. The best Rowan could do with her, I think, is to secure her neutrality in case of a power struggle inside the castle. (-)
* X'zaratl: I agree that X'zaratl is a trusted ally of the Twins. Despite having some sort of sympathy for Rowan & Alexia, she is a demon and siding with Bloodmeen's current rulers seems like very obvious thing for her to do. It could be extremely hard (if not impossible) to win her over. (-)
* Shaya: Shaya is Jezera's close friend who shares personal history with her. Betraying Jezera is not an option as for now, maybe much later in the game, if Rowan successfully completes her veil/true identity quest and manages to break through the barrier of her "selfless courtesan" persona. (-/+)
* Skordred: As a loyal servant of the previous ruler of Bloodmeen, Karnas, Skordred will not betray the Demon Lord's offsprings. (-)
* Draith: Although Rowan can make him his bitch, I don't believe Draith has courage big enough to stand against the Twins. Alexia can catch him red-handed stealing drider eggs, so maybe Rowan will be able to somehow threaten him into helping. If so, then Draith could unleash driders at a critical moment during the coup d'etat, greatly increasing Rowan's chance of success. (-/+)
* Nasim: Similar to Cliohna. He is very independent and doesn't trust anybody. Could be a shaky ally but wouldn't take much risk as he doesn't seek political power or wealth, only to further his knowledge of magic. Maybe Rowan could promise to give him more resources for research in case he takes control of the castle. (-/+)
* Indarah: It doesn't look like she cares about politics or power. She seems very focused on her personal life and job in tavern. (-/+)
* Delane: If rescued by Rowan and not turned into a mindless slave, she becomes his lover and is thus extremely loyal to him. (+)
* Ameraine: We know very little about her motivation. I don't think Rowan could count on her help, even if he gets closer to her. (-)
* Fae: Gaining help from the Fae will be extremely important for Rowan. I believe that in the later part of the game Rowan will have to decide who to support in the ongoing struggle between the Midnight Court (Arzyl) and the Red Sun Alliance (Whitescar). From already present conversations with both ambassadors it's clear that Whitescar, despite being wild and brutish, is not evil and doesn't really like the Twins. Arzyl is much more like Jezera. I think that the choice who to support among the Fae will determine whether we can later count on their help. The Red Sun Alliance could help Rowan, the Midnight Court - not necessarily.
* Bloodmeen's main army: It's mostly made of orcs and demons, so I think it will remain loyal to the commander-in-chief, Andras. Maybe some troops that consist of human mercenaries could support Rowan, but that's only a smaller part of the army.
* Allied orc army: Whether these forces support Rowan or not will probably depend on who is their leader. So the ending of Orciad quest will prove most important here. We must compare all four possible outcomes of Orciad. It's obvious that betraying Tarish by helping Delane escape will make seeking her help impossible. It's also obvious that Batri is loyal to Andras and doesn't think highly of Rowan. Ulcro on the other hand, is more sly and less of a brute than Batri. It's made clear during the celebration party in Bloodmeen that if Ulcro wins, Delane becomes his chief advisor and makes decisions for him in all political matters. Rowan is only able to support Ulcro by gaining her trust earlier so she could be open to repaying him later by swaying Ulcro to Rowan's side. If Rowan helps Tarish while not saving Delane, he gains her trust and even makes out with her. So, out of four possible outcomes of Orciad, two can provide Rowan with an opportunity to seek support of the orc army later.
* Allied goblin army: ??? We will see when this quest is implemented. But it's reasonable to think that, similarly to Orciad, it will give Rowan opportunities to gain their leaders' trust while forging the alliance.
* Conquered Rastedel: This will certainly depend on who becomes the city's new governor after the takeover. Whereas Patricia corrupted by Rowan is connected to him personally, Patricia corrupted with the amulet and Ameraine's help is 100% loyal to the Twins. If Duke Raeve, who is Jezera's pet, becomes governor, Rowan won't get much chance to gain any ground there. On the other hand, governor Jacques, while distrustful of Rowan, doesn't seem fond of the Twins and could possibly ally with Rowan again, provided eventual gains outweigh the risk.