The RF input does support different regions yes. I think it’s only SECAM that’s not supported.
No, you can only use one or the other of the expansion cards at the same time.
No you can’t use the PSU from OSSC classic or RetroTINK although these are the same voltage they don’t output enough amps. The remote can be re-used if you don’t mind losing some of the OSSC Pro specific buttons.
As for the RF version it looks like maybe bundling it with a high quality (2x shielded) RF cable might be enough to clear up the interference so it could still happen.