As none of you knew, the 101 GBA I got had a couple of issues - hardly responsive left shoulder button, and later on, and almost unresponsive A button. I figured I'd have to resign myself to just finding another one some day, but then I remembered that I have basic soldering skills.
I pulled a shoulder button switch off of another GBA that I have that I got years ago that has some crappy aftermarket case replacement (seemed like the perfect donor), and swapped those out. That part was easy, since they're through-holes on the board. The A button was another story, since that's an annoyingly thin surface-mount microswitch:
It appears that the GBA was the first and last handheld (not sure what the Switch does) to use microswitches instead of the usual printed contacts with the rubbery button pads that everything else uses.
Getting the switch off was interesting. Luckily I didn't have to salvage it, since it ended up being partly desoldering, and partly pulling it apart accidentally as I desoldered.
I had found some replacement microswitches that would do the job on eBay. Like $4 for 50 of them. A little smaller, but their leads lined up with the soldering pads just fine. Here's one of them, next to a regular SD card for size reference (on the board in the first picture, this new switch is about the same size as the ones they used for Start, Select, and brightness):
Now that it's in, my A button works again! It clicks a lot more than the stock ones, and the button doesn't have that loose jiggle that the usually do, but the important part is that when you push it, it clicks and does what the A button should do. Hooray!
Oh, and ...
Sounds like a fun game try it
It's not. It's some dice game, and when I tried to play it, I flailed around doing nothing until I got into a screen where I had to move dice between my pouch and a "pool", and I couldn't exit out of there since I had no more dice in my pouch, but I couldn't leave until there were 15 in the pool. It's clearly a game you need the manual for.