MY partner and I faced much the same dilemma before our trip for a safari out of NBO about a year ago. Our questions were the discomfort of BE going via AMS (Yes, we're spoiled. After many RT's in "real" FC on JAL, Singapore, Malaysia with flat beds, didn't relish 20 hours in cradle.)

We ended up going JFK-LHR-JNB-NBO, the transatlantic on UA (no DL partner seats available, but for $220 or so flew UA RT in coach)(BTW, the trip was a birthday present for my partner. Even with super cheapo tickets and only Gold status on UA, they upgraded BOTH of us to FC when I told them it was for his birthday...JUST LIKE DL WOULD, right?

The SAA lounges in LHR were quite lovely...we spent a 12 hour layover (after great food and shower in the lounge) noodling around London. HUGE flat beds and superlative service on 12 hour flite to JNB...then a wonderful (not as fab as MQ in KUL, but pretty perfect) SAA lounge and a thoroughly delightful flite to NBO.

flew back from Mombasa via NBO (I highly recommend a couple days post safari R&R there...we took the o/n train NBO-MOM just for the experience and it was a great, almost anachronistic experience...

Happy to provide more info and details offline if you'd like.
Steve