VMs, I reckon that has a good chance of working. SPi isn’t very resource hungry so I could imagine the performance being good. I’ve used VirtualBox for years for work, and it’s solid. Not tried it for audio though.
Re SPi as a midi generator, control source. Yes, yes, yes! Great minds think alike I know, it’s fantastic for that. I keep banging on about it in fact. Forget your DAW in session mode or BeatStepPro with its three sequencers - SPi does the lot. Myself, I run a hybrid using a mix of sound from SPi and other sound sources. There’s lots of really useable synth voices, and the sampling powers are excellent (endless really).
Re your point about the dummy sound card - what I did was to have two USB sound cards attached (or use the PC’s headphone jack and one USB sound card) and then have SPi use one output, and the other sound sound use the other. You do need an audio mixer, but then that’s similar to having two separate boxes.