Let me try answering your questions:
It is technically possible to pursue any of the five characters on any route, but that doesn't mean it'll always end well. And yes, on the A route, it'll be difficult to spend any time with Maeve (for obvious reasons) so their dynamic here takes a bit of a different turn... So to answer: yes, it is possible to romance her even if you're enemies, but it won't be a very traditional sort of romance progression.
To your second question: First the story splits into A, B, C, and then it splits again depending on which character you are focusing on, so there are 15 routes in total (like A-Vynn, or B-Maeve, or C-Lester, for example). As for different endings, there will be 20.