Whilst lots of trips are done across Africa in 2 wheel drive vehicles, you may struggle a bit crossing central Africa. 1) regardless of what vehicle your in, its a problem at present because of the trouble in Chad / Darfur in Western Sudan, CAR and Congo. Several travellers have headed down the west coast towards Angola. 2) Anything's possible but this is probably where you are going to struggle. Going down through west africa should be fine. tarmac nearly all the way if you want it and lots of 2 wheel drive cars go down on things like the Plymouth - Dakar. Again possible on the east coast although your immaculate BMW is going to get badly worn in the process.
Re carnets - few options - 1) take out an insurance policy but with double indemnity. Basic options are a) put up the money, b) insurance policy with single indemnity which removes the need to put up the money but where the insurance company will still chase you for the money if they have to pay out, c) insurance policy with double indemnity where they pay out and don't chase you. Have this is on the basis that you genuinely lost vehicle - eg stolen etc. If you sell it and then try and use the insurance on the indemnity, then you are commiting fraud. Insurance company will expect to see paperwork such as Police report supporting fact that vehicle has been stolen.
2) meet the vehicle import requirements for the country that you want to leave the vehicle in there by releasing the claim on the carnet. 3) don't use the carnet in the country you want to get rid of the vehicle in - eg use it up until that point but get rid of the car in a country that doesn't require the carnet. Quite a few of the east africa countries will use it if you present it but don't actually require it. 4) get a fake carnet but be aware that if you get caught, they can take the car and put you in jail. Unlikely to happen particularly if you have a realistic looking carnet (obviously helps if you have seen a real one to compare with because some of the customs officials will have seen quite a few, especially in east africa and just because yours has lots of stamps doesn't mean they can't spot its not real if it looks different to all the other ones they see.
Good luck with your trip