At first I thought the game might get too repetitive with the same gameplay of rescuing students. But in the end, I enjoyed this game thanks to the more interesting boss fights. The visuals are very nice and I liked the story and concept, it definitely stood out to me. As for the questions you asked:
How do you feel about the 3d movement/shooting mechanic?
I got used to it eventually, but throughout the game I found the height of bullets to be unclear.
If you have to play procedurally generated chapters repeatly, do you feel if it's repeative?
I'd say yes, unless you manage to add more variety such as new enemies or stage hazards.
If it's a local multiplayer game, do you think it's better to keep in one view and it should split to multiple view?
I'm not really an expert on multiplayer, but I'd prefer split screen over having the camera locked to one player and have everyone else have to follow that person.
What elements do you want to add in the game?
Stage hazards, more enemies, more types of bullets to dodge, maybe powerups...