Thanks for trying out our game!
Biboo's attack is a LoS check, the 4 vents are there to block the attack. The sheathing sound after the cuts signal when its free to come out. (sadly its very quiet and godot didnt really let me make it louder, i will need to enhance the raw audio fiel instead)
4 bosses clearing stage 5 is a bug caused by the system that handles fuwamoco. (If fuwawa dies first Mococo gives up, but if you defeat mococo first it counts mococ twice, adding up to 5 boss deaths) ITs already fixed in the devbuild.