I have thought about this.
Ok now consider this: warmest pants I know of -Nunatak Torre Bibs have 1.5" loft and 11oz fill. FF Expedition Down suit has 26oz fill and I suspect 1.5" loft.
From Hal Weiss's Secrets of Warmth:
Effective Temp Sleeping Light Work Heavy Work
-50F 4.0" 2.1" 0.52"
So far my argument against it was that it will be too warm for pants even though the only extra weight is a zipper. But climbing summit in blizzard? You have a point there :)
You have summarized well all the ideas discussed here.
I think Mode 2 will be very drafty so extra fabric will be required to seal the bottom.
Now the cost:
Torre Bibs and Jacket cost $1150 and are less warm then this design. FF expedition suit costs $950. I am not sure if nunatak or FF will make you this under $1000. Why should they?