If your character is adjacent to only 1 enemy fig, can you use RCE on it?

Only if either you or your opponent are fliers, ( or both).

If your fig is a flier, then yes... fliers can always make ranged combat attacks.

If your fig is a non-flier and the only opposing figs are fliers, then yes... a non-flier can't make ranged attacks if adjacent to an opposing non-flier.

Also, you cant RCE an adjacent flyer who is standing in hindering terrain and have stealth or Batman TA

Or you have trick shot.

Simplified answer:

Yes, if a) either you, the target, or both fly and b) you can draw a line of sight to the target.

Remember, you don't need LoS for punching, but you do for shooting.

You cannot use Trick Shot on an adjacent figure at all. Trick Shot is negated if you are based by an opposing figure.