The Governor couldn't do it alone because the 1961 Constitution was an Order in Council, and one specifically authorized by Parliament at that. So Parliament would have to pass something like the Southern Rhodesia Act, 1965, which IOTL allowed the British Government to legislate directly for Southern Rhodesia (which it used to dissolve all Rhodesian government institutions). Of course, since UDI had already happened, it was basically a dead letter.Will? Yes, it was what both the Conservative and Labour governments wanted from the early 1960s. The noisy opposition from the far right reaches of the Tories notwithstanding.
Ability [to impose by military force]? Not after UDI, no forces that could invade Rhodesia or bases. Beforehand, maybe but highly uncertain.
The Governor-General could dismiss the government elected on a White Supremacy mandate by the restricted voters roll. But I’m not sure if he could then legally impose direct rule through the civil service and impose a new constitution with a broader electorate (majority rule). Which would be what is required.
And “legal power” need not equate to actual power to enforce matters. The issue was discussed at a seminar I attended for an Open University history degree module, with the interlocutor using extracts from British government documents. IIRC there were concerns that parts of the local police and military would not follow the orders of the GG in such a scenario. At best a sort of Curragh mutiny, with resignations and obstruction by some locals in the civil service and other government department. At worst a coup by Smith’s supporters and possibly outright civil war..
A lot of British government documents have been put online. Certainly for the 1930s and earlier, there might be ones released for the 1950s and 1960s too by now. Could be worth searching for them.
Under parliamentary sovereignty Parliament certainly has the legal power to impose majority rule. But the Rhodesian government controlled its own military. This and the distance from Great Britain make imposing direct rule more difficult than it would be for Northern Ireland.
Just like for the Confederate States, UDI was a mistake since the UK could do little to actually impose majority rule.