I would focus on using AA miles for IAD-DEL if possible (as above, search ba.com for QR and CX availability, and use Etihad's website to search EY availability). For the rest, look to United - TG flies DEL-BKK non-stop, and United has lots of partners that could fly you BKK-IAD. Another good option for BKK-IAD is via HKG and another US connection on CX/AA with AA miles. What cabin are you looking for?