The ones in one of the DMGs (green) looked a little fucked up, but when I spun them and everything, nothing happened. And the other DMG's (yellow) contacts checked out the same, but those actually even looked pretty much normal, but it happened when I tried both gameboys during the show.

Clean the battery contacts.  Pop them out of the case, soak them in automotive battery cleaner for a little while, scrape them clean.  If the Gameboy spontaneously shuts off when you touch the batteries or spin them in place while it's on, then you definitely need to do this.

1) the USB wall charger devices should work just fine.  they are still the same voltage as the port on a computer, you just get more juice from them for quicker charging. 

2) the cart's battery is for maintaining the data on the SRAM IC in the cartridge, it wouldn't cause the gameboy to shut down even if it went totally dead

1.) Does anyone know (presumably the person running the kitsch account on here) if it's safe to use kitsch's USB-to-DMG charger by plugging the USB end into a USB-to-wall adapter of some sort, or should it only go into a laptop? It just seems like it'd be more convenient to not have to plug the Gameboy into something that, itself, might need to be plugged in.

2.) An unrelated question: Recently during a show, my DMG shut off spontaneously--sort of. I think the Gameboy may have shut off for a split second, but either way then LSDJ disappeared, yet the battery light was on and there was a black horizontal line on screen. I replaced the batteries, and the same thing happened. During a dry run before I went to the gig, everything went fine. My assumption off the top of my head is that it's the cart battery. Yes? This has happened before I think, but it seems to only occur during shows with the gameboy playing at high volume...