Keep in mind that ScanSnap manager still works in Mojave. So there is still a year in which to complain before you are stuck with the ScanSnap Home.
I can address 5.a, b, and c.
5.a. and b. You “sign out” on the old computer first. If you don’t you can file a ticket with Fujitsu support and they will reset it for you. Note that there is no provision for borrowing the scanner – you may be restricted from signing back in on the original computer.
5.c. Easy – see above. Attaching the license to the new computer is automatic – you don’t even realize there is a license until you try to add a second computer.
If you want to be able to sign in on multiple computers you would need to buy a license, but there doesn’t seem to be a way to do so for the iX500, just the iX1500 (which comes with a 5 computer license).