- Yes, but not impossible.
- The only one who dropped the ball is Apple, who prefer creating standards rather than adhere to them.
- You seem to have a short memory. In 2007-2009, Apple used Nvidia and AMD. 2010-2011, AMD. 2012 was Nvidia. 2013 had some products use AMD, some Nvidia. 2014 onwards has been AMD. No reason Apple would lock them into one GPU provider (and Intel). It's not in their interests.
- They pulled staff from other teams to help with the 2016 MacBook Pro, presumably delaying everything else.
No reason why Apple couldn't use Nvidia in the MacPro7,1. As an example, a mobile GTX 1070 has a TDP of 115w; the current 450w PSU could easily power two of these, a Xeon and whatever else. And that's not factoring in advances in SFX PSUs in the last 4 years.
What sucks is that they went for some bespoke connector rather than MXM. But then I guess they'd only do that if they wanted people to upgrade, which they clearly don't any more.
Apple are clearly done with doing their version of a classic Workstation design, and the commodity-style miniaturised form factor is probably here to stay.
The gift and the curse of going with Apple is they're at the bleeding edge; you can get features and a sense of design that's way ahead of everyone else, but there's always a cost in one form or another.