Crescendo over a tremelo

I think you might need to add a second Voice with all the notes in it plus a starting and ending dynamic then hide them. As is repeatedly stated on this forum, MuseScore is primarily aimed at creating great-looking scores and playback is a secondary feature to enable you to do this. Having said that, it does a fair job of it IMHO.

You can't in the notes that you are displaying, but there is a work-around:
In the Instruments dialog window, create another Instrument of the same type that plays the tremolo.
Back in the score, set the original tremolo notes to not play, by using the Inspector.
In the new instrument, select the measure of concern, and enter (instead of tremolo) rapidly repeated notes that mimic the tremolo effect (such as a series of 32nd or 64th notes). Apply a crescendo mark and either put a piano and forte dynamic before and after the crescendo; or use the Inspector to add a velocity range to it.
Confirm that the audio playback is what you require.
When you have the playback to your liking, go back to the Instruments dialog and un-check the box that allows the new instrument to be seen in the score.
That should do it! You can *see* the crescendo/tremolo entered properly in the score, but you *hear* the measure in the staff that isn't seen.
I hope this helps,