Admin vs User Privileges
I'd suggest MuseScore should check for certain files and directories (e.g. plugins, instruments.xml, ...) in two locations, putting the more recent into effect: on linux, for example, /usr/local/share/mscore/ and /home/username/.mscore/.
In this way, plugins can store or change things even when the programme itself is installed outsie the user's home directory.
Hosam Adeeb Nashed