I just got back to spend a little more time trying to narrow down where the problem is... in doing so I found out that mame64 isn't launching roms anymore, it just quits mame when I try. What on earth did I screw up?
Edit: If I open Mame64, go to Configure Directories, Select Roms, and then exit back out of the menus without changing anything, roms will once again launch from Mame64's UI. Then, if I open Attract Mode, try to launch a game (which instead launches Mame64's UI) and I try to select a game out of the Mame64 list, Mame closes. It'll continue doing this until I go back and do the Configure Directories thing.