so, if people have better ideas where which should go [i.e. is a Player Piano a mechanical instrument first, or a keyboard instrument...?
Why not put it both under mechanical instruments, and under keyboard instruments?
When using hierarchy browsing you will then find the songs tagged with it under both nodes.
But, I do set Instrument tags by hand, as for me the important bit is the song defining instruments and NOT every instrument used in a song or state in the credits - so the musicBrainz instruments tags would only have limited use for me anyway. On top of that, I find them a fairly patchy mess...
Wow, that's a lot of work.
I feel that if Picard/MusicBrainz would work better for you that might be an easier path.
You could then add instruments for recordings to it's database, and you would get those, and the ones that lots of other users contribute back through Picard.
And you would help improving it's database.
I do share the idea of having a 'main instrument' for a recording though.
For most classical concert pieces I don't need to see all the instruments in the orchestra, but only 'Piano' or 'Violin'.
I do have a custom tag for that. But that's a manual process.
(I also use that tag when e.g. a band has a brilliant bass player or contains a great bass solo)
But MusicBrainz is also not ideal to be honest.
Not all instruments will get their own unique entry, but are added as sort of a comment/refinement that is impossible (or just difficult, I have to check again) to retrieve.
Also, if you would like to suggest missing instruments to be added to their database, I find the person responsible for adding instruments (the instruments inserter) not very responsive or approachable.
So I have not been very active on that department myself to be honest.