As to the difficulty of playing other instruments with the organ, with very few exceptions I don't think I've ever heard/seen that done in the few places where an organ was available and might have been used.

The organ was presumed to be a "complete" instrument, and usually or often accompanied the choir; but good choral harmony generally is pretty much in "just tuning" even if it's an accident (although not as close as Barber Shop?) and voices are so flexible that it wouldn't matter a lot except on a chord-by-chord basis.

If instruments were used, it was usually a solo or small ensemble performance, and the organmeister got a chance to take a nap. If a larger group of individual instruments played, it was an "orchestra" considered "complete unto itself," and again the organist took a timeout.

There have been special events, when one of the classical orchestras helped to demonstrate a new organ by playing along (usually because of a new home for a restored classic organ?) but does someone know of a common usage of the sort, perhaps historically, where the organ played with the orchestra as a regular form of performance?

My curiosity is just whether it really happened very often.

John