The first assumption is wrong. Vodafone only needs another 1% of Vodacom to have effective control of it. And Telkom will not be happy with owning less than 50%. IMO that is where the negotiations are failing so far: Telkom wants to get rid of all 50% at as high a price as possible, but Vodafone is not willing to buy all 50% at current rates, or even a small discount (Its between paying $300mn and $10bn for essentially the same thing). There will most likely be a large BEE component in any deal - the only question would be what price do they pay for the shares.
Once Telkom sells their 50%, they become a takeover target if they dont get rid of the cash soon. One option is buying back shares, but even buying all their shares on the market, Telkom will still have huge amounts of cash on hand. Another option is acquisition of another telco/ICT company in SA or more likely in another African country.
MTN will only buy Telkom if it is in their long term interests, not only in their SA operations, but in their other African operations too. This may be a possibility if Telkom were to make major acquisition(s) of a fixed line operators in other countries in Africa (such as Nigeria). Otherwise i dont see why MTN should be interested in buying Telkom shares except to use Telkom's cash to pay off their own debt.
Telkom may be better suited to buy CellC (which could use a cash injection), giving it control of a cellular operator in SA, as well as pursuing investment opportunities elsewhere in Africa (fixed line and/or cellular). This scenario establishes Telkom as the 3rd telco operator behind Vodacom and MTN, with Neotel effectively becoming the 4th. I dont envision another competitor in this market, though there will be a buhch of smaller players providing different, more specialised services to the market such as IS, Verizon, VoxTelecoms, municipal networks, etc.