I can’t get physical copies new of just about any game made before the late 00s.

Hm… it’s not best practice, but that shouldn’t kill it entirely either. Ah I wish you were local, I’d love to take a look at it.

Did you disconnect the battery without shorting the two terminals together or using a metal tool? You should never ever unplug anything in a device until after you’ve unplugged the battery.

No. Opening a device is categorically NOT a warranty voiding action, by law. At least in the US. Though, in this case, it does sound like they damaged the joycon, which would make it not eligible for warranty, but it has nothing to do with opening the device.