to run the TJ wheels you would have to get an adapter to go from the TJ pattern to the JK pattern...........most 15" wheels will fit on a JK but some require grinding the caliper to fit properly.
depends on the backspacing.
but yes you can. I have a friend with a JK who runs 15" soft 8 steel wheels, with no grinding
Backs will clear, up front must run 3.75 backspacing, as said earlier, get some adapters, might not have to run as much backspacing with spacers