I was editing and expanding my reply ..... it should address those points now. Following your original suggestion of the copy shopdata code it took a few days... I didn't realise at first the second comment was from yourself also.
Server settings were part of the problem. Log was not useful as it would sometimes attempt to work on a table, other times drop it due to timeouts. Once settings allowed the "upgrade" to "complete" the backend was non-functional. It would appear that the 500 errors were result of corruption from insufficient resources under server settings, which is why I have listed everything that resulted in a partial successful upgrade.
not reduced rights, the core admin account has no access to the backend after 1-click upgrade.
Superadmin is part of the quote from the quoted thread, along with the solution.
all files are 755 directories, 644 files etc as per an usual installation.