Wow, so many options with your wealth of miles! Can I borrow some? I'm an American living in Kenya so I have already done a lot of the research you are seeking, and I'd be happy to help.

I would recommend that you do your trip round-trip United business class for 120K miles each ticket, which you have easily (with plenty of backup by way of ultimate rewards). This gives you:
1) Great routing rules flexibility with 1 stopover and 1 open-jaw. You can do a stopover anywhere in Europe, Middle East or Africa.
2) Lots of carriers and routes to choose from. Your best bets for 1-stop routes (excluding Egypt Air) are on Turkish through IST, Swiss through ZRH, and South African through JNB. If you are into trying out cool business class products and don't mind a 2-stop itinerary, you could piece together something with Austrian's new 767 business class (onboard cook and milkshakes!), LOT, and/or TAP.

To explore potential routes and stopovers on your own, go to the Star Alliance Route Map (http://routemap.staralliance.com/). Click on Nairobi to see what your options are in/out of NBO, and then piece together your ideal itinerary from there.

I don't think trans-pacific routing is possible to/from Africa on a United award--I have tried searching it but it has always returned an error message.

Today I just booked a one-way, 8-leg business class ticket from SFO-NBO with 20- to 23-hour stopovers in Sarajevo, Munich, Zurich, and Tel Aviv. Looking forward to visiting friends and seeing new places!

Lastly, I have never pushed my blog on FT, but this post actually may be very useful for your situation: http://www.pointsforgood.org/?p=568

Feel free to send me a PM if you have any questions. Good luck!