Survival after 1979 could have been possible, however white Rhodesia was basically done for by 1975. The biggest factor leading to Rhodesia's demise was the Portuguese withdrawl from Mozambique in 1975. Though there had been guerilla incursions in 1966, they amounted to nothing. In 1972 and 73 there was some guerilla activity from Zambia, but again the border with Zambia was defensible. The Portuguese withdrawl from Mozambique gave ZANU and ZAPU bases from which to operate and the country was surrounded. Keep Portugal in Mozambique and you've got a white Rhodesia surviving.
Then there are the economic problems. Though facing sanctions, Rhodesia's economy was able to grow steadily until the oil crisis hit in late 1973. In fact the sanctions actually helped the economy diversify and become less dependent on tobacco exports. Trade also diversified and was no longer as dependent on Britain. White immigration increased to the point where there was a net 15,000 in 1972. Though there had been a blockade on the Beira pipeline since 1966, oil was still able to reach Rhodesia mostly from the port of Lourenço Marques by rail. The OPEC oil embargo targetted Rhodesia and in 1973 Ian Smith travelled to Portugal and in meetings with Portuguese premier Caetano and other Portuguese officials apparently the Portuguese were willling to supply Rhodesia with Angolan oil refined in the Lourenço Marques refinery. However, in April of 1974 there was a coup in Portugal and the government changed.
So if we can have Portugal stay in Africa. I would suggest a government that is willling to abandon Portuguese Guinea (where the guerilla war was mostly lost by 1973) and stay in Angola and Mozambique. This will insulate Rhodesia (as well as South Africa). Though it is unlikely that sanctions would be lifted, if Rhodesia can last to let's say 1982 it has a chance of the Thatcher government implementing a solution akin to the Pearce Commission of 1971 since the conservatives will just want to rid themselves of a 17 year old problem. Also with Reagan instead of Carter in the Whitehouse there won't be too much opposition from the Americans.