This itinerary seems perfect for a *A itinerary using UA with an allowed stopover.
IAD-XXX-DEL can be LH/UA, even ET. DEL-BKK on TG or AI. BKK-IAD - mix of options on UA/NH/TG/etc.
If DEL-BKK gives you fits on miles, just buy the one-way, and use UA miles for the rest.
Last edited by aacharya; Aug 8, 2016 at 11:36 am