The characters are zombies, limbs and heads sometimes pop off, spin backwards, eyes pop out. The closest thing to consistent body horror is Junko being seemingly Frankensteined together but this is never shown in detail and she doesn't actually look that strange.
Nothing actually happens but in season 2 Saki (18) briefly develops a crush/feelings for a guy in like his 50s. He tells her she's too young for him and its made pretty clear it wasn't actually about her liking the guy.
its very subtle, but when the girls first "wake up" as zombies you can see shots of characters like ai and junko looking into mirrors and not really "seeing" themselves. this sort of goes away after their first arc though
Characters mention having muscle fatigue but its a one off line and its not clear if its a permanent thing for them, a permanent thing for zombies, or just a typical thing for idols because of their training
not really, but theres a misunderstanding that makes kotaro bring an unconscious girl to the house. it mostly just results in confusion and everyone apologizing