That was a good read. I am not expecting such low temperatures during the day (just when it gets colder at night). With a loaded bike I may need to walk up some hills, otherwise one gear would be okay.
I've never had to do repairs, but that definitely sounds like a difficult task, can't imagine taking out my tire from the rim (especially if its frozen). There seem to be towns every 10-15k for a good portion and at most 30-40km in few areas.
And for the water: in a thermos I was able to keep it for 4 hours (and it was still warm when I finished my water), even with colder temperatures I am hoping to be able to store some water in it between towns.
I'll definitely look into those hand warmers you mentioned in your century ride story, they seem like a great idea. (so is the cream)