I'd try AA metal into London. In order to fly to JRO, you have a few options. BA can get you to NBO, and then take Precision Air into Arusha. Obviously, BA has the huge surcharges for that one leg. The other two carriers into JRO that might work are KLM from Amsterdam and Turkish Airways from IST. Don't know if you have enough Star Alliance miles to do that, though. As you noted, Qatar does fly in from Doha, but award seats are very few.