Surprisingly, this mostly solved the issue. Just blew into whatever little space there is around the L button and it's mostly working now. Who knew the NES blowing technique still worked on their newer devices in 2017? Thanks for the suggestion.
It was the B button from skipping so many useless dialog boxes in Animal Crossing New Leaf because they don't have auto-forward features for its text boxes like many modern JRPGs and such. Metroid also has this issue, but luckily has less dialog/text boxes to cause issues.
I thought about the New 2DS XL, but the color options are terrible. I'm not a fan of the two-tone designs they've announced so far, as I'd prefer a solid-colored design with black like the previous 3DS models offered.
Woo! So glad that fixed it! I'm sure I've said before that Mario Kart and Street Fighter II blew out the R button on my SNES controllers.
All of this made me buy, and try, some compressed air on my fancy Samsung robot vacuum that was malfunctioning. It has a "bumper" too, and just like Frisky's L button, there must have been something stuck under there. It's back to serving me without complaint. I'm glad, cause my floors were getting kind of bitty.