1. The evolution of man and the evolution of technology is what invented and created the wormhole. In reality man is always evolving and our technology is constantly advancing at as of right now a rapid pace, things that even just 10 years ago we could not imagine be possible is what we view as the norm right now. Eventually we will master the mysteries of the universe and soon will manipulate it at our own will. In THIS film in particular, once Murphy figured out the blueprint of defying gravity, which is essentially the element of the 5th dimension it mapped out the beginnings of redefining the universe and the advancement of mankind along the way. "They" = us in such a way that we damn near evolved into Gods.
As for which branch of humanity it is, it's not known, but my best guess it's the colony that Amelia started.
2. Interesting thought. Being that the 5th dimensional beings do not have the concept of linear time because they view the past, present, and future consecutively, it is very well possible that the whole reason why the worm hole was opened was for Cooper to travel forth and back. This is clearly the reason why it was possible for himself to "handshake" Amelia as he was traveling back when they were traveling ahead.
3. Who knows how it would be like, but I best guess it would be best for them to be kept separate. That way, the introduction of racism, prejudice, and bigotry wouldn't interfere and the colony start fresh without any pre-influence of their society.