I'd start rereading anon, but here are some answers to your questions>why does he need to figure out who is who?
Partially because, whether he's aware of it or not, he's embarrassed about his inability to tell one from the other despite the fact he has an involved relationship with the quints. Consciously it's because he wants to be able to figure out which quint is pushing him away.>Are there several sluts with wounds on their legs are is it one klutz who has all the wounds all over her legs?
Both Ichika and Miku have bruises on their legs, the former from some accident while hiking apparently, the latter from the altercation with Fuutarou.>Whats the point of all dressing up as the same girl if the granddad allready can tell the difference between them and realize they are all different individuals?
The grandfather interprets the quints looking really different from one another as them fighting, he apparently got ill with stress when he thought his granddaughters weren't getting along- so now they dress up like one another when visiting to make him happy.