Regarding your 2nd question. I've found that the original control unit has some big advantages which is why I use the "add-on" setting with my DKC-850 and DKC-500RW xg.
1. The ensemble/accompaniment tones/sounds are much better than the DKC850 produces.
2. If you record you can layer up to 16 tracks of piano and ensemble sounds.
3. Calibration, testing, and having a DK tech do diagnostics is more complete.
4. If you have a PRO model the fidelity of the PRO recordings you provide or from sources like Yamaha's e-competition are much better.
5. If you have a Silent model the digitized acoustic grand piano accompaniment is of a Yamaha Concert Grand sampling and is far superior to the acoustic grand sampling included with the DKC850.
6. Having both head units set up this way also allows 2 stereo outputs so you can layer accompaniment/instrumentation from both units out to a stereo amp or multiple amps/monitor speakers etc. Example: I can have the DKC-850 putting out harpsichord and the DKC-500 flutes and acoustic piano simultaneously.
I'll have to search back from prior postings, but I believe there are a few other advantages to setting up your Disklavier in this manner. For those who really want DK-Radio and TV, it's simply moving the main cable and one setting on the 850 menu and you're done.
Lastly, Yamahamusicsoft dot com is running their Valentines 20% off sale on all Pianosoft music, not a bad deal.