Is it possible to install & successfully fire a 7.62x39 upper on a regular AR15 lower? If so, are any modifications needed and would it be a good permanent upper per for that lower?

Yes.

Really depends on the upper. Several company make 762x39 uppers for the AR15 platforms. Still really not a question for a Turners rep tho. Like James said.

I don't know if Turner's would be able to answer the question you may not need to talk to a gunsmith some of guys that turns your wedding audible some guys don't know a lot just depends