largely agree, even in the 1990s the white South Africans owned South Africa total, what broke it was the start of boycotts and sanctions to bite average Whites, plus the end of the cold war was the total end of the idea of US or UK support and also the end of the idea that ANC victory might lead to "Southern African Soviet" a taste of terrorism (and it was only ever as taste of it) also scared people about the idea of living like the Israelis, the Whites didn't have the will to fight for apartheid, but the blacks didn't have the strength to win such a fight or even wage it
You're mostly right on this one - the wrong part is the US or UK support. London and Washington had no love for apartheid, and Pretoria had little love for Washington and absolutely none for London - the memories of Lord Kitchener's camps where 26,000 women and children perished from disease have lived long in South Africa. What ended apartheid was the economic issues. The ANC's moving into terrorism against civilians - which it should be pointed out a lot of ANC commander did not agree with - was not a huge factor, the big one was the economics. By the early 1980s, some farsighted individuals in SA realized that apartheid would one day fall, and they wanted to know that they, their people and their investments would be safe. The ANC had figured out by this point that they had little hope of physically defeating apartheid, and that getting an agreement which would see apartheid end would be worth compromises to some of the white elite. The USSR supplied guns and some money to the ANC, but not that much, and Moscow was not about to blow up South Africa, as it would cause problems for them as well as for the West. Botha's attempt at accelerating Verwoerd's plan of grand apartheid and the 1983 Constitution was an attempt to head off the falling of apartheid at the pass.
that said, what would 20 years of political, diplomatic and economic total isolation do to the SADF and the government and average white citizens? if by the late 1980s the ANC was willing to plant bombs how much worse would this get as hope about the fall of apartheid fades, as hero figures like Mandela die in jail and younger more fire breathing leaders rise....
Mandela isn't gonna die in jail, as Pretoria was well aware that that happening would blowup much of the nation. As for diplomatic and economic total isolation, the former was unlikely and the latter impossible owing to what South Africa's largest sources of revenue were - exports of metals, many of them critically important to the world's economies. That isolation was what the SADF dealt with in the 1980s, which is why South Africa's arms manufacturing industries got to be as advanced as they did - this country made its own small arms, armored vehicles, trucks, transport helicopters, anti-tank and anti-aircraft missiles, towed and self-propelled artillery, missile boats, a supply ship, attack helicopters, MRBMs and nuclear weapons. They license-built attack aircraft in considerable numbers, did major upgrades of aircraft they already owned and had a fighter aircraft program well underway before De Klerk canned it in 1992. There really wasn't much the SADF couldn't do themselves aside from main battle tanks, and considering they developed their own turreted, rather advanced AFV (the Rooikat), I don't think a real MBT would be beyond their abilities if it was needed.
As far as firebrand leaders go, that is a potential problem on both sides. Some of the whites, including a lot of De Klerk's government, realized the need to negotiate out the end of apartheid, but we both know that there were lots of whites who continued to feel that the
baaskap, the mastery of the rest of the peoples of South Africa, could be maintained if they just killed enough of them to cow them into accepting white rule. (The fact that sooner or later people in such a situation just keep fighting because they have nothing to loose and don't give a damn anymore never seemed to occur to these people.) The absolute worst case scenarios would see SA bring its colored and Indian populations in with the whites (in such a conflict, they'd get shot at as often by the blacks as by the whites in all likelihood) and securing the major cities, pathways to the ports and the mines, and leave everything else alone - and if that means the outlying whites have to become paramilitary loonies to survive, well, so be it. That would be horribly damaging to South Africa's economy, but if its that or losing a chance at survival, they'll fight viciously. Apartheid could have lasted to this day, but it would be a worse place than South Africa is now.