As it's a non-harem game, will the other LIs still remain close friends with the MC even after you're on a specific route? Because a mistake I think many AVNs like this do is make the other LIs treat the MC basically like a stranger if you're not on their path. Which kinda becomes a disservice to the characters.
Yes!
That's one of things I'm trying to improve on with the current reworking as well. At the moment, you can say Mc is friendly (and doesn't seem to be a reason not to continue) with Avis/Koko/Raiko, but much less so (depending on choices also) with Serena, Levy, Valentina and Eiko. It was a bit unclear from my own previous writing, but it should be showcased a bit better in rework.
So yes, MC will be friendly with all of the girls throughout the game and continue interacting with them, and some choices/pathing will be slightly shrunk to better portray it (like removing not tripping thief which locked you out of any meaningful Levy talk for 2 updates).