I’m assuming the problem is that the version of the game or neogeo are not matched to the version of MAME. The thing is, the trial and error is taking forever and I think I’m even redownloading some ROMs from sites I’ve already been to. Is there a smarter way to go about this?
In case it’s still needed, or if someone needs it, a suggestion that could work in OpenEmu (can’t test as I don’t have a Mac), try putting neogeo.zip of the correct region in the same folder as the ROM. Usually emulators can find the Neo Geo BIOS that way.
And a tip, most modern PC releases, including iirc the Amazon Prime bonus, come with both the ROM and a matching BIOS. At worst, you’ll have to extract the ROM from the Linux build with 7-Zip or similar (points at GOG). And those PC releases, they’re dirt cheap too, specially when discounted. Metal Slug specifically, 1 to 3 and X all are emulated, and further titles already on PC, I have yet to confirm.