Yes, at times I have 3, one iron, 2 VMs. Testing I might have 4-5 machines running, the one iron, the rest VMs or Pis.
Yes, I test all the alternate OSs on VMs. I have a 14TB NAS I use for storage so no issues for space, YMMV.
Absolutely, if the cameras allow multiple connections concurrently. I share the USB cameras from the iron machine too, with no issues.
Good questions.