Soundfonts Are Screwing Me Over, Please Help

That link doesn't seem to work, but perhaps I can clarify a few things:

musescore.com only supports the default soundfont - it does not / cannot maintain copies of every soundfont someone might want to use instead. So anyone playing a score via the online player at musescore.com will hear the default soundfont. Anyone listening using the mobile app will hear the soudnfont that comes with the mobile app, which is different. Anyone listening by downloading on their own computer will hear it using the soundfont they have chosen as their own default.

Which is to say, you can only control how a score sounds for you on your own computer - you can't control how it sounds for anyone else (except by creating a videscore and encouraging people to listen to that rather than use the online player or download the score to their own computers and play it using their own preferred soundfont).

Not sure what you mean about instruments muting, but perhaps you have tried using a non=-standard (not GM) soundfont and are trying to play it using a GM soundfont or vice versa. Hard to say without seeing the actual score and knowing what soundfonts you are trying to use.

It's not entirely clear what you mean, as the link is generating a 404 Page not Found error at MuseScore.com.

A couple of things Marc missed, however.

If you are using a custom soundfont regularly, you should click on the "Set as default" button at the bottom of the synthesiser dialogue once you have installed it.

This will ensure that it is loaded for you next time you start MuseScore, and that instruments you have set in your score remain consistent.

The other thing is to do is click the Save to Score button, which will automatically load a particular soundfont when you open the score.

HTH
Michael