@Nadine. Your original post talked about loading fonts to and from scores and why you had to do some of that every time you opened a score. If you use the sounds that came with MuseScore, you hardly have to mess with any of that. Let's say you have the sounds that came with MS set as default ( done by selecting the font in the Zerberus tab in the synthesizer and selecting "set as default"). If you start working on a piano piece, you use the mixer to assign a piano sound to the piano staffs. When you are done working on the piece for the day you select "save as", name the piece and save to a location of your choice. That saves everything and all settings you created. The next time you open that score everything will be loaded just as you left it. Sounds, reverb settings, everything. Then, just "save" after subsequent sessions. You don't have to load to or from anything, unless you decided to change fonts. Then you have to do it all over. Every time you start a new score you must assign sounds. MS can't do it because it has no idea which of the violin sounds you want, for example. If you open someone else's score it might load sounds for you if the score uses the same font you have. I have never used save to score, or load from score or whatever. I think you may be making this more complicated than it needs to be.
Other news is about your piano piece. Quite lovely, by the way. I know you stated why you want to use "fp". I get it. But "fp is not possible on a single note on piano. More proper to use two separate markings right where you want them. Just for the fun of it, I tried loading your piece into Sibelius to see if it made an audio file any faster. But Sibelius can't read your repeat structure. There may be some disagreement but it's always safer, when you repeat back to the beginning of a piece, to have a repeat sign at the beginning also. Sibelius when to some random spot 10 pages prior. Just a thought.