Basic understanding of soundfonts and Musescore staff "Instruments"
I am using Musescore 3.6.2 and have succeeded in a score creation process where I assign "Instruments" to staves and can hear those instruments playing the score through the "mixer" window and can then even export as an audio file with those same sounds--all without ANY external hardware like MIDI.
Wanting to augment some of the basic sounds I had at my disposal (which I assume are all derived from the default "MuseScore_General.sf3" soundfont?), I tried dabbling in adding additional 3rd -party soundfont files. I succeeded to the extent that I can, in fact, under the Mixer window, select a given staff instrument slider (like Electric Piano, say) and "Change Instrument" in the mixer to play a selection from the new soundfont file.
What is confusing is that I thought I would also be able to select a staff in the score and when requesting a "Change Instrument" from there, be able to choose among the "new" instruments from the added soundfont file. However, that does not seem to be the case. Is there something more I need to do, or am I misunderstanding the nature of Instrument assignments on staff lines versus what is available at the Mixer window?
Part 2 of my question is I started all this hoping to find a true stereo Fender Rhodes keyboard sound (which auto pans volume L to R as a real instrument can. I realize this was probably simple-minded thinking as--unless the soundfont instrument included a separate sample for EACH note in the range--polyphonic playback would probably produce a very strange panning effect with different notes in the range panning back and forth at different speeds. So my question is: is there another mechanism under Musescore (something like a "plugin") that could made to apply to an instrument at the "Mixer" stage that could add stereophonic effects like that to a given mixer feed?