Can you clarify exactly what you're doing with the 'thumb drive' ?
It sounds like you're using the old EVRSI activation method and storing the activation file (EVRSI.SYS) on a removable USB storage device 'thumb drive'. You might remember using the EVMOVECF utility to place the activation file there.
RA has been very slowly migrating away from EVRSI to FactoryTalk Activation, which uses a plaintext *.LIC file that's created to be tied to a specific attribute of a computer, like a hard drive Volume ID, the MAC ID of a network adapter, or a special hardware ID of a Rockwell activation 'dongle'.
That RA activation 'dongle' might also be thought of as a 'thumb drive'; the newer ones also have some storage onboard.
For a very long time, PLC logic editors like RSLogix 500 and RSLogix 5000 have supported both EVRSI and FactoryTalk activation methods.
RA finally stopped supporting EVRSI with RSLogix 5000 v20, but I think that RSLogix 500 has retained the ability to use the old activation method.
At this point you've done a bunch of uninstall/reinstall and registry cleaning, so you may have done more harm than good.
Your first step should be to determine which activation method your older computers are actually using.