I think the で is indicating the method/way the 5 levels are determined.
The examples don't have an embedded question but, here's some with questions:
I think replacing で with の ...
This English translation is correct at least in that ～でも means "even (in/with/etc) ～". This 初めまして同士 means "two people who met each other for the first time" (or maybe "two first-timers" depending on the context). The first half of the sentence basically means "Even though it's a pair session of two people who met each other ...
Yes, the second じゃない is an informal way to say something like "right?" or "don't you agree?". You can see the rest of the sentence as the actual statement, and じゃない just as a way to involve the other person. Something like "isn't it?", if you want to have a negative example.