Agree with bypassing China, Tibet, and hassles/cost structure of latter.

From Denver, I think easiest way to get to KTM is going to be via DL/KE:
DEN ->SFO or LAX -> ICN -> KTM, with the first segment on DL and TPAC + intraAsia on KE. Possibly need a long layover or overnight in Seoul due to connection time, which might be kind of nice way to break up the long journey. My personal next best choice (Star*) would be DEN (UA) ->LAX -> BKK ->KTM on TG, but that ultralonghaul has been quite pricey this year. With an extra stop, UA + TG can get you to BKK via SFO-HKG, which often works out cheaper.