Hank,
You have a very good analysis.
However, SG cars could not easily make the old RGS curves and clearances. I would suspect that a new SG grade would be built if it could be economically justified.
If I recall correctly, there are a couple of short lines that reach north of the old AT&SF east of Gallop. If so, one of those could be extended or connected to.
I also think that building a new SG line up from Gallop would be the best instead of down from Montrose. Just like the one that the AT&SF was rumored to want to build that caused the D&RGW to respond with the Farmington branch.
Doug vV