{ Ref. Ed Copeland in 200228 MMDigest ]

Hallo Ed, your question is a classical one!  Getting an answer
immediately generates at least two new questions.

The field of [application of] MIDI for reproducing pianos has
been present in MMD since nearly the very first days.  You can
scan the archives in order to find the information you need:
https://www.mmdigest.com/archives/KWIC/M/midi.html 

As an alternative, I suggest visiting the web site of Terry Smythe,
now accessible under  http://www.terrysmythe.ca/rebirth.htm 

Next, proceed to the section about "e-Valve Equipped Pneumatic
Instruments", where a lot of your questions are probably answered.
One of my own contributions there is titled "What Is An E-roll?".
I highly recommend reading it.

Best regards from Germany, where the corona virus has even won over
the Brexit.

Walter Tenten