It is definitely possible to get from Jo'burg to Malawi by public transport. There is some sort of express bus that takes about one and a half to two days or so including border crossings. Many Malawians who work in South Africa use this service frequently.It is also possible to take different busses the entire way stopping off wherever you want to. I don't think it's possible to jump off in Mozambique, though and it's important to have your visa in order for the Tete corridor crossing to save hassle and time. This is definitely the prefered route to Zambia as the roads there are something to behold! Most people who go to Malawi plan to stay a few weeks and end up staying months. Mulanje, the islands, the northern lakeshore, Nyika, Livingstonia, etc. are all famtastic. But I would stick to the northern part of the country if your time is limited. The people are friendlier and the landscape lusher. But the entire country is very nice. Of course I'll have to give Mwaya a plug, too. www.africaguide.com/mwaya A great place to stay on the way to Nkhata Bay. All the best in your planning and have a wonderful trip.
Pitani Bwino.