Air Cars was apparently some kind of encrypted cartridge or something based on the comments in the softlist. Even with the checksum hack it didn't do anything.
Regarding stuff breaking from the PACK fix, it's highly likely that there's an interlocking set of bugs that made some things appear to work and that fixing only one of them fixes some obvious test cases (e.g. Chequered Flag) and breaks others. We see that sometimes on the MAME side as well.
