I am in favor of selecting routes that put you in the right place at the right time, so I would warn you away from riding in NM, AZ, and UT in summer. Those states are best in spring/fall. Springtime in the desert is amazing - cactus flowers! You could probably make it work, but being off road in the desert in summer... you would have to take the heat/water situation seriously.
Have you considered the great divide mtb route?
http://www.adventurecycling.org/routes/greatdivide.cfm I think they have a lot of info about which direction/when to do it.
There's no reason a long bike tour has to start and end on the coasts. You didn't say anything positive about the eastern riding -- so, you know, you don't have to do it.
There's a colorado trail to, I think it's called, uh, the Colorado Trail. I'm biased, I like Colorado. And the West. West is Best.