I glad to see someone else as interested in playing the African campaigns as I haven't ventured into any of the other campaigns to date. I will offer, as some have offered me, that once you complete (or tire of) the South Africa mission that you try one of the lessor regions.
I have to agree with tboko that massing a hard, rolling attack against an opponent will surely limit the war if done in a manner that will cripple him. For me I have found that taking his resources that I can get with minimum loss is a first step; capturing as many towns with minimum loss (by use of airborne, SOF, etc forces); destroy key bridges to me is helpful to funnel his forces; using my air force to destroy any and all of his attacks will buy you huge gains (be prepared for many repairs); and having a wealthy number of artillery pieces.
For Namibia I found it beneficial to surround his capital and the other large city (Harare) while taking the other cities as quickly as possible or blocking them off from being a threat to my forces. This would also divide any valuable property from Angola and the other countries in the event that Namibia did surrender. By choking the capitol and other key cities with large troop buildups while I seize the other cities and resources I effectively minimize my losses while ensuring that I obtain a large portion of the enemy's troops. Usually you will have destroyed all of the major combat assets (tanks, etc) but there are usually large numbers of support units (air defense, arty, SOF, etc) that I usually sell for a quick lump sump of cash. Each country will continue to get more difficult as they will continue to get more and more WM offers while you continue to get less and less. You will notice that the AI ran countries will equal or surpass your military in numbers while their economy and and population will not be able to handle such a large military. You will have to get over that flaw in the game and continue to kick their asses! Planning on your part will surely gain you more than a simple "move out and kill everything."
In my post for this game I eventually took over Africa while maintaining a, albeit smal, WM relationship, continued to have allies until pretty much the end of the game, and was successful in 80% of my decisions. The 20% error came about when a country would DoW when I didn't expect it to do so and I was unprepared. This resulted in me lossing territory and bases and almost all of Niger, however I quickly regrouped and while Sudan was focused on clearing me out of Niger I simple moved from the North, East, and South of him and eventually took Sudan and reclaimed Niger.
Back to your desire for the AI to surrender. . .I thought that I had this figured out many times to ensure their surrender to minimize my loss and maximize my gain. Not to mention my thinking that a "quick" war with minimize loss to lives and damage to cities would also minimize my WM opinion loss. I would not count on too many of the AI surrendering until you have taken them to the last the city! Tanzania is a good example in which I have had to take him to the very last city everytime.