possible spoilers here
I don't think we will have to choose one person. Hyao would mention it as a future feature in the game if it was because of how important that will be at the end. we prolly can have multiple relationships up until the end, but at the end I believe we will prolly choose one person and that would be the ending and credits would roll or we start a mega harem. also, everyone would cry if we had to choose one lmao.
this is also something I don't think is likely, Hyao wouldn't just force Bernard to be the final love interest, he just seems like the most likely and the one with the most romantic scenes since he's been around from the literal start. I think it's a choice between whoever we want at the end if anything like this.
this though, this seems REALLY possible. I have never thought this up but considering the MC's personality if it came to it, he/she would. I just doubt it would be the canon ending, Hyao really likes giving us balance between funny, serious and depressing because if it was only one of those the game would begin to feel more like a game built purely for people who like horny/cute, horny/serious and horny/sad causing the only good thing to be the sex and feel we've seen it far too much. plus, only killed Gunnar because it was required for plot/character development with MC, Seth and Tank while also warning us people CAN die and no one is immortal even with our intervention.
also we do know one of those questions answers. that one being the reason demons and shit are interested in us. it's because we keep becoming the center of attention everywhere we go, but started when we met nameless due to us being the only person not protected by Orsu and Razit's blessing making it so he could manipulate us into doing as he asked since he couldn't do it himself and that we thwarted his original plan to corrupt and kill them all to steal the gems after the temple fell. everything else we don't know though. all we do know is literally just she/he's been working for his/her master for a few months and what we make him/her become.