Can I, at some point in the game, turn against the twins and ultimately defeat them?
Not yet implemented. The game is a work in progress;
however, there are certain actions in the game where you can undermine the demons and Bloodmeen even in the current state of the game. I haven't played the latest build(s), but there were a number of things you can do in previous builds and each, small success is a minor victory that ultimately moves towards this goal (if you are so inclined to take this most noble path).
@nazg2001 Since I've never thrown Alexia to the wolves to see the event where that scene comes from nor do I have the game currently unpacked, I can't be one hundred percent positive about this part. I believe that first scene you posted with the image only occurs (minor spoiler warning) when events take place so that Alexia gets involved in trying to help Helayna of R.Keep to escape Bloodmeen. When the Twins demand an explanation, Rowan turns over Alexia who gets sent to the dungeon (and initiates its own scenes with, uh, a certain cute creature). Eventually, Andras will come for her--there are eleven CGs in total around that.
For the second one (Greyhide x Alexia), it occurs after Rowan meets with Greyhide and asks him if he would want to meet his wife. It chains through a couple other events including that threesome scene you've mentioned. At some point, Rowan and Alexia thinks about their relationship with Greyhide. You'll need to choose to continue seeing him. Alexia needs to work in the Forge for a bit and eventually that scene will trigger. Short disclaimer, I haven't played this game since at least February and trying to keep track various things in here and elsewhere may cause some of it to drift in my memory, but this should be (about) right. Either way, hope it helps and that you're having a lot of fun with the game! Enjoy!