I can't say definitively for all the questions but digital volume control is a bit of a red-herring.
Meridian use a hybrid approach of analogue and digital volume control. Anything with a variable analogue output (including DSP speakers) will be responsible for varying the gain in both analogue and digital domains.

So:
1. Master DSPs act as the reference point for the other speakers in the DSP setup, so your slave speaker(s) are set relative to the Master for volume control.

2. If your M setup is done properly, no changes are made to the digital volume until it reaches a DAC.

3. If correctly configured, the device set as 'Controller' instructs all other devices over comms as to the volume they should set (if they can). This gets confusing with the surround processors and DSP speakers, as you have a Master speaker and a processor as Controller.

4. That would depend on the configuration.

5&6. I would say a MC200 as just a Core doesn't need to be Comms'd to a system connected to the MS200. A MC200 used as as endpoint, connected to say DSP33s, would need to be in order to control the volume using the Control app.



Mark
G68, baby Genelecs, Humax Foxsat, Oppo+Vanity93, Toshiba HD-A35, Pioneer CLD-925 + Meridian 519, various other random bits and bobs
Got a black G08.2 you want to sell?