In #1 you indicate arrival NBO 10;30m and in #3, indicate layover 5pm and depart 8am assume next morning... which is it - 10:30pm or 5pm?
Regardless, unfortunatley NBO/JKIA isn 't the most pleasant airport to spend overnight. Few comfy places to sit, few services as restaurants but for the Java Coffee House and lots of shops. While some who layover during daytime hours will pay a small fee to use one of the lounges (believe $30/pp for 4/hrs), don't believe these are open 24/hrs.
So whether it's 10:30pm or 5pm, best you arrange with tour operator for a meet/greet and transfer to hotel in NBO for overnight, then transfer next morning for onward flight to Kili/JRO, TNZ. Or you can take a taxi with cost of about USD$20-25 depending where your hotel located payable in Kenyan Shillings to/from the airport. It's perfectly safe to take a taxi.
Nearby hotels to the airport are Panari or Ole Sereni,, both of which tend to be expensive, or the new EKA Hotel where price should be lower. These 3 hotels are about a 15/min drive from NBO/JKIA. For more competitive hotel rates, consider the Fairview or Country Lodge and though a greater distance from/to airport, if your flight to TNZ is 8am, you have to be there 2/hrs min. prior departure... so there shouldn't be much traffic at 5am.
If overnighting in NBO, you will have to purchase Kenyan Transit Visa at USD$20/pp and require Yellow Fever inoculation and proof of same for entry into TNZ.
Regarding the balloon charge in Serengeti, while this activity is more expensive than if in Kenya, $570 is way too high. Usual price is $500 ($525 would be maximum).
Edited: 6:07 pm, January 16, 2013