Sounds good, but I don't think the Di2 rear derailleur works without the front derailleur tied into the system...
Of course, I suspect that means you could run a mechanical FD, plug-up the Di2 FD and strap it to the water bottle cage and the RD wouldn't know the difference.
Again, the stoker Di2 remote control doesn't strike me as all that bizarre and completely workable with a good team that communicates well. However, I'd love to see / hear how it actually works. I found I had to re-learn the timing of my shifts to get clean gear changes with Di2. Perhaps having your stoker call out shifts will actually create that slight timing delay that you need with Di2?
As for the rear disc, it'll also work... to a point. On the bright side, since both Kent and Katie are professional off-road level cyclists as well as elite roadies I would guess they know how to manage brakes well enough to avoid over-using the rear disc and/or the rear rim brake. So, it will probably work for them. That said, INMO the Bengal or Avid with a 203mm rotor would have been a better choice.
Thanks for sharing the new insights.
Awesome frame.... and it's our size !!!