Making somebody your ally who is willing to betray their master is completely different to simply gaining that person's trust. I agree that Skordred and Cla-Min can become close to Rowan, but I don't think this is enough to secure their support in case of an open struggle between Rowan and the Twins. Like somebody said earlier, probably the player will eventually be able to sway to his side almost every NPC in the castle. But that's in the long run, only in the much later part of the game. I don't think Rowan could gain much support at the end of Act I.
Also, SoC will probably be similar to BioWare's games, where certain morally-aligned decisions divide your party, making some of your companions support you and others oppose you. I'd be willing to bet that gaining support from every NPC in one playthrough will not be possible in any case. Winning over some of them will require decisions that isolate the others.
Regarding X'zaratl and her obsession with Rowan & Alexia... I have my own theory about this. I don't believe we can trust X'zaratl 100%. True, she seems trustworthy and wants our protagonists to love each other even more, but at the same time she is a succubus, a type of demon specializing not only in sex, but also in deception. Whereas in terms of gameplay X'zaratl's scenes serve as a counterweigh to large amount of NTR options, in terms of her role in the plot I don't think she could join the "good"/Solansia side. All her events give the player big amounts of corruption points. This is just my own theory, but I suspect X'zaratl isn't truly in love with Rowan & Alexia, she is just playing with them, introducing them to new fetishes and willing to see how far will they go. Her plan is to corrupt them, not to help them.