It's been a while since I owned my VR-76, but I would think that if you connected the footswitch to the VR and midi'd the keyboard to a controller, then you could switch the leslie speed that way.

As for your other questions, I don't think you can use any two SRX sounds at the same time, as you are limited by using the synth section just once, correct? If that is so, then the answer to the other questions is NO.

Do you have a manual? If not I think I have a PDF and some other stuff.