There will be a GUI, yes - for testing it's easier to use a CLI program though.
We do intend to be able to have more than one Prisma card. Probably the first version won't support it but we'll see how many people want to run more than one at once... it's probably not terribly useful though as most people only listen to one music track at a time

I suppose it could be useful for DJing, though, to have one fade into the other....