Is any of this applicable to home recording and people listening on their phones? I'm totally lost. I'm not sure what the difference between M-S and X-Y is.
Wikipedia actually has a bunch of articles on stereo mic'ing, and they're pretty decent. I recommend diving into that rabbit hole. In a nutshell, all stereo microphone techniques involve compromise in one way or another. X/Y is one that is pretty neutral. M/S has a single case where the problem becomes, IMHO, catastrophic if you need the stuff on the extreme ends of the stereo field. Don't worry though - you can't do it by accident.
Panning won't create a problem on it's own. The real issues arise when you have signals that are within a few milliseconds of each other, and they create weird phasing problems when combined. It's actually the same effect you'd find in a guitar phaser pedal, but unpredictable and jarring.
And yeah, actually, Bluetooth speakers are a really good example of a modern listening device with mono output. A lot of them are stereo, but the little ones are typically mono, and will absolutely cause the same issue.
The big thing is that all this stuff is why experienced recordists will tell you to "always check your mix in mono." It will always sound a little different, but you also want to make sure it isn't creating odd phase-ey things or making things stand out or disappear in ways you didn't expect. It doesn't mean you should mix with the assumption that your listener is using mono, but just make sure it's not offensive.