I'm no authority on O-roll matters but can perhaps contribute a little
as I have built an O-roll machine. It's still running after 20 years. As
to your questions:

  1. Both the Principal and Secondary instruments use the same range of notes.
They're not alternate sets because they can play at the same time.

  2. I don't use a piano damper during solos. My machine has no pipes.  I play
the piano along with the bells and it sounds OK--the bells drown out the
piano.

  3. Same answer as 2.

  4. I have a North Tonawanda band organ which plays Wurlitzer 150 rolls.
I am adding a set of bells to it. Last summer I asked David Stumpf,
a band organ music arranger, if the pipes should be shut off when the
bells are playing.  He said no. I also have a home-made version of the
Wurlitzer Caliola, to which I have added bells. The pipes continue to
play when the bells come on.  It sounds fine. The Caliola uses Wurlitzer
Caliola rolls which have the same scale as the Wurlitzer 65 note
Automatic Piano  rolls, but are scored specifically for the Caliola.

  5. O-roll arrangements are famous for their great piano and percussion,
including bells.  The pipes are not great.

I'll watch your progress with interest. -- Fritz Gellerman