Can't you just expand the setting in the Score Manager and divert your "changed to" instrument to a different VST and instrument?

Yes you can. There are two problems with that. 1) Finale loads new samples into the next "free" spot of a player. If you have a previously designed set of sounds within certain players and Finale "doesn't know" about this, it simply overwrites a slot.

For example, I have a meticulously desingned soundset in ARIA that uses all 16 Channels. When I now make an instrument change it overwrites one of those channels instead of opening a new player and using a free channel there. I've tried many times.

It only works if you initially chose your sounds not by loading a complete soundset into your player, but by adding one sound after the other thru the direct dropdown-menu inside the score manager. When you chose ARIA player and then inside the player load a sound set that has the Flute in Channel 16, Finale might overwrite that slot with somethign else you chose via instrument change. However, if you use the instrument directly from the list, say JABB->Woodwinds->Flute Player 1, Finale will not overwrite it.

I sometimes use 3rd party libraries, though (like Kontakt 5 sounds) that need to be loaded inside the player and don't show up in the score manager's dropdown list. 

Now problem 2). When I set up my Big Band instruments directly thru the dropdown menus, and use doubling instruments that I chose from that non-editable list, it loads the default settings of those instruments. All settings like controllers, EQ, PAN, Volume, etc. are reset to default and the there is no way I can load "my" Clarinet or Flute sound that I have edited to my liking since Aria doesn't allow the loading of separate sounds, only sets.

So, I start my BB piece, I make an instrument change from Alto to Clarinet, I have to edit the Clarinet sound from its default settings by hand. When I have another sax later on in the piece and need to change to Clarinet, the same thing happens - manual editing, again. But it get's worse: The only way to get rid of instrument changes is by clicking the "X" in the score manager and deleting and unloading the sound completely, along with all settings. So if I change my mind mid-piece, I'll have to do all the diting again...

It would be so easy, if in the score manager (or ARIA for that matter) one could make up ONE set of instruments that load at the beginning of your chart, including all settings, EQ, Reverb, EFX, controllers, etc. Now when you'd do an instrument change, Finale would simply change the transposition (make it visible, too like with staff styles) and chose the sample from your preloaded list and boooom.

Or, alternatively, Finale would not have a fixed list of changeable instruments, but another score manager just for those extra instruments where you could either decide where Finale loads them. The simplest way to do this would probably be to load them into a new player, where they would reside until you tell Finale to unload them.

You appear to have moved the goalposts since your initial request. I hope MM can satisfy your request down the road.

"Or, alternatively, Finale would not have a fixed list of changeable instruments, but another score manager just for those extra instruments where you could either decide where Finale loads them. The simplest way to do this would probably be to load them into a new player, where they would reside until you tell Finale to unload them."

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PLEASE make the "change instrument" feature editable.