1,2, and 3 are all essentially the same question. ST hasn’t announced how virtual devices will transfer to Edge - they can be created in several ways (official and unofficial) and the answer may not be the same for all of them. They will most likely continue to exist but may not function as expected if they’re assigned to a driver that doesn’t have the same capabilities.
3 - if the old virtual devices are deleted, your automations that rely on them may go away with them.
If you don’t have too many, since it sounds like you’ve just started setting them up, the easiest thing would be to create new virtual devices on Edge and swap them out in your automations.
4 - There are a number of different virtual device drivers available, some for specialized purposes. For general use, here are a couple of popular ones:
Some general reading on Edge and the transition if you aren’t fully up to speed yet: