Point by point:
1, Yes, but as soon as a recalculation that involves a 'premium' feature is required, you will get a red.
1. Yes, but see point 1 above.
1. You can have different versions of Prime side by side on the same computer. That includes Express. But I don't think you can have Prime 5 and Prime 5 Express on the same computer. ( unless multi-boot).
IF....you own a license it is easy to switch from Express to full Prime. To switch back would probably require reinstall or you'd Have to invalidate the license file.
1. See point 1 above.