well i think that the way you have it written is fine, as you have a lot of characters so killing tham off with stronger ones is fine as long as you have a way for the MC's party to come out of that island fine. maybe even pick up a couple of other ppl along the way. I wouldn't have a very big party to leave the island besides the instructors.
Most likely yes. I plan to rewrite a certain part of the island test before continuing. Or perhaps I will change some event previously planned. I think one of the mistakes I did in EM was introduced too many super powerful characters too soon