OTL, 11 November 1965, the white-minority government in Salisbury issues the Universal Declaration of Independence of Rhodesia. The next day the UN Security Council condemnes the "illegal racist minority régime".
But despite NIMBAR the door for compromise was still open during the 1960s and Mugabe, etc. hadn't yet as much political influence as after the war. So, what if Ian Smith is replaced by someone, who is less determined to maintain the entrenchment of white minority rule in the constitution at all costs, but someone who realises that Southern Rhodesias demographics make a whites-only government impossible in the longterm.
So instead of fighting a long war Rhodesias new PM negotiates a power-sharing constitution with moderate black leaders and includes them in his new government. Maybe something like Zimbabwe-Rhodesia, with two distinct electoral rolls and a large number of seats reserved for whites and maybe additional generous minority rights. Together with the economic advantage the white population had and the fact, that most of the black population lacked any formal qualification, that would have secured white dominance for a long time.
Would such a settlement have been accepted at this time? I think yes, as the Rhodesian government was at a much better bargaining position in the 1960s than they were in 1979. And Britain was eager to release responsibility for this embarassing situation. So Rhodesia becomes an independent Commonwealth Realm with a black prime minister but politics and economics are still dominated by whites.
As this isn't a strict one man one vote solution, some kind of nationalist insurgency will probably still arise. However, international and internal support will be much lower.
But will this arrangement be stable? More influence for black politians will certainly mean more expenditures for black education, etc. but when they realise that the white will be unwilling to give up their privileged position for real there most definitely will be a backlash. More radical politicians get elected who will challenge the power-sharing agreement which privileges the white population. The insurgency will become more intense, though still not reaching OTL levels. But it still means a strong military presence, and a predominantly white officer corps.
So Zimbabwe-Rhodesia gets a quite radical prime minister, who excludes whites from his government and instead invites the insurgency parties. He introduces policies of land redistribution, declares his intention to change the constitution to remove white privilege, ec.. Remember, we're still during the Cold War, so the Soviet Union is most eager to use the opportunity to gain a new ally and supports his new government financially. Naturally, that doesn't go well with the white population and a military "anti-communist" coup ousts him and installs a white-led military government. Some token black ministers are included, but martial law is declared, democracy is swept away and policies are designed to provide for the white population first.
Could this be a realistic timeline? And how would the foreign relations of this regime be? If it avoids to be overtly racist and considering that the USA supported Pinochets coup against left-wing Allende, how would the western world react?
But more interesting: How would such a scenario influence the policies of apartheid South Africa? The crippling sanctions came only at the end of the 70s but apartheid policy created strong international backlash even before. Would the contrast of a more democratic and stable Zimbabwe-Rhodesia intensify the international condemnation? And would it influence the internal policies of South Africa?
Of course, South Africa will support the white-minority military dictatorship. Considering the different timeline they are not convinced that Rhodesia is doomed to fail, and instead of using it as a bargain for themselves, they try to secure its existence as an important buffer state.
How plausible is this time line and what would be the long term consequences. I appreciate your comments.