stage directions

To say always use xxx text would be reckless. There are too many variables. Writing "pianist grimaces" is perfect for either staff or system text depending on how many parts you think need to see the note. If you choose staff text you may want to copy it to the gunslinger's part so he knows that happens just before he shoots the chandelier. If several performers need to know this happened so they can gasp, then perhaps system text would be better suited and you can make it invisible to the musicians who are tacit. The question is which would be easier for you.

If you need to explain the scene in some detail, then a frame of some sort would be the logical choice. Consistency on the part of the engraver/composer is paramount here so when the director sees the text he knows its scope. Once again, if certain performers don't need to see the detail, then you can make it invisible in their parts.

There is a certain amount of correlation between what is standard notation and MuseScore's options but it's ultimately the composer who must choose the best tool for the situation, just like choosing an oboe or violins to play the melody. MuseScore gives you plenty of options. I've seen people use rehearsal marks in rather ingenious ways when they aren't using them for anything else.