On the bags, would it be overkill to have a 32 degree bag for the high elevations in the West and then ship it home once into ... Kansas? We will have friends sending us re-supply items via General Delivery as we go along, so I am thinking a swap to a "lighter" bag could be accomplished.
Makes sense to me. If you have a bag shipped to some place like Boone, CO, you can stuff the colder bag into the same box, tape it up, and send it right back. (I only mention Boone because we had a care package waiting there!)
Whether you need something that warm depends partly on luck -- you might have 45 degree lows all through the Rockies, or it might frost. Leaving early July, you've got a good chance of hitting the east coast by the end of September, and a 40-45 degree bag will probably be fine. You may spend more time sleeping on top of it, for padding, than in it for warmth.