Still trying to divine exactly what the problem is; I can believe it may be SQL is installed, but I'd also like to find something more definitive to confirm that rather than just take it as a wild guess/assumption.
Digging in the C:\$Windows.~BT\Sources\Panther\setuperr.log file the first line says "InfEnumCallback: SetupGetInfDriverStoreLocationW failed for 'C:\Windows\inf\oem5.inf' [gle=0x0000090]". It happens about 8.5 minutes before the upgrade process actually err's out, so not sure it's a real issue. The rest of the log is filled with error messages basically identical to what this other poster posted: https://social.technet.microsoft.com/Forums/Windows/en-US/b380ed57-a437-4372-bf8c-a94054c49ca4/upgrade-issue-from-2008-r2-to-2012-r2
He says "... there was some orphaned drivers lying on the server. after removal of them, server upgraded successfully.", so I'm assuming the same may be true for me but he never says what he did to find/remove the 'orphaned drivers'.