In case someone ever finds this thread, I will just provide brief closure, with more info later in my "live report" thread from the trip.
Yes, Kenya is perfectly self-driveable, yes you do need a 4x4 for most concievable destinations - even in parks that are ostensibly "accessible with 2wd", the roads are amazingly bad and chances of killing a normal car are astronomical. Yes, you can camp in all KWS parks and some more reserves and yes, wildlife will be there alongside you in the night but it seems to work fine.
Renting a 4x4 in Kenya is weird though. We went with a Hilux from Roadtrip Kenya, which had good reviews and unlimited kilometers - at the end of the day we have driven only 5300 kms in 25 days which wasn't THAT much and some other company with limited kms would have worked out some 15 percent cheaper, but I do not regret having the freedom of mind not having to plan for kms.
Roadtrip Kenya is a really friendly company (well, it seems that it's just two or three guys), they provide you immediate support even with non-car related things - they are basically your friend in Kenya you can trust. When we had a breakdown in Mara, they arranged all help we needed swiftly and we were back on the road by evening at the cost of $230 from my pocket for the repair (as they claimed that it was caused by me forgetting to disengage 4x4 on a road). After returning the car, they found two broken pieces of leaf spring on the rear axle, considered charging us for that (at 50 USD, so quite reasonable) but decided not to considering how much money we paid already for the long rental.
The only downside really was that the car was old, had 250k kms on it already, had no A/C and some "non-essential devices" were not in the best order, lot of sources of noise when on a corrugated road etc... Personally, this is the kind of car that I am used to drive and if a company offered this to me at a discount, I would take it, but at 139 Euro per day it just feels expensive for what you get.
On the other hand, apart from this one breakdown (which may have had been my fault or not, hard to judge for me) and the broken springs (that were not felt during driving and caused no issue), the car survived incredible things with ease and went through any track I could find. The tires in particular were amazing, surviving direct hits with acacia thorns, but also the 3 liter engine was well kept and really smooth.
The guys from Roadtrip explained to me that this is simply the economy of rental in Kenya. I do not have enough information to verify that and thus on a next trip (should I ever do one) I would probably try another vendor just to get some perspective. However for a one-time visitor, I would in fact recommend Roadtrip Kenya, they have simply proven to me to be reasonable and it's quite possible that the others aren't better because it's not financially feasible. It's just important for any traveller to have the right expectations to avoid disappointment.