Last time I checked it, it is a part of the game that hasn't been developed yet, the whole "choosing who sleeps in the same room with Rowan" can't be freely changed at the moment, if you let Helayna stay, she will be your roommate for the rest of the game so far, which means that every scene that could mention Alexia being in the room with you will instead say she's somewhere else.
I don't think it will actually block any content with Alexia provided the rest of your stats with her are good, so I don't know what you mean with "forgive you", she won't necessarily be mad at you, but she won't sleep in the same bedroom and that's all I can really say.
Now, to speculate about the future, there will probably be the possibility of harem/polyamorous developments in the future, and I don't see anything that would block Alexia from being a part of that (specially a corrupted Alexia that loves Rowan). The devs will probably eventually add the ability to choose who is sleeping in the same bedroom as Rowan, and of visiting his other romantic interests in their own bedrooms if they aren't sleeping in the same bed as Rowan. When that happens, they might add a scene with Alexia deciding how she feels about all of this (based on her stats) and she either accepts and joins in, or puts some distance between her and Rowan (something like staying a wife just in name, but with both of them pursuing other people).
