Reading your excellent description/quandary and sharing much of it(!) it comes to mind selling both the 7000 and 300 replacing them with a 400 would solve a few problems
I think we can pretty much guarantee now that the EN-EL15 will be in the D400 and that it will share the hyper FPS of the D4 and the new AF advantages of D4/800.
So a D800E/D400 end state allows the D7000 or 300 to just be an interim and may make the choice easier. Hmm for you it sounds like D300 is the way to go. For me I am hoping, next year mind you, to keep my D7K and IR convert my D300.