[mhauru]

Agreed. Although as you point out, the singers being unaccompanied is important. Mixing keyed (or fretted, valved etc.) instruments into the ensemble quickly starts to push everyone towards 12-EDO.

I also wonder how much continuous pitch instruments do this, when playing pieces of music and with ensembles that would allow adjusting to just intonation. If I walk up to a good violinist and ask them to play a simple melody on a single string, do they gravitate towards just intonation, or does muscle memory make them place their fingers for 12-EDO? What about a superbly good violinist?

[genericguy]

I'm a violinist! The answer is "it depends". There are so many complexities to tuning that relies on harmony which is continuously changing. Mostly it is done with careful listening and intuition. Even playing with a piano, we won't play in equal temperament 100% of the time.