One of the things that is not readily admitted to by those in the white community who lust after the so called good old days, was that civil disobedience amongst whites was growing. In particular, response to compulsory military call ups for active duty, were getting poorer and poorer. It should be remebered that SA had been fighting a low scale war since circ 1965; This had escalated, by the 80's, into a full scale confrontations between the Cuba/USSR/Angola/SWAPO/ANC axis on the one side, and SA on the other. The war involved the full spectrum, from MBT's to supersonic jets to submarines and littoral naval warfare.
The costs were really hammering the SA economy - sanctions didnt help, but in my opinion it was the cost of the war that really hurt. Also, after 1976, the internal security situation was parlous to say the least. Parts of the townships (Dormitory towns for blacks, generally situated out of the white cities) were ungovernable; The homelands or bantustan policy was failing and white emigration was skyrocketing.
So you have a demoralised or antagonistic white community, sanctions and a war to be fought and on top of it, internal unrest.
I dont believe that De Klerk was any more of a liberal than any of his predecessors; He was however, a realist and the writing was on the wall for the apartheid state. He did what was right for the country.
Apartheid could not, and in fact never could have, survived. If the decisions of 1990 had not been made, the result would have been very very ugly. For one thing, the regime fielded the most powerfull and technologically savvy field army in Africa, supported by a powerfull airforce and a large, and brutal, police force. (The police were militarised, and had their own armour and airwings) There was an advanced NBC program and an armed civilian population. There was also a very clear set of war plans. On the other side there was the makings of an internal insurrection, ill equipped, ill led and without clear war aims.(Kill the Boers does not constitute a war plan when put into action against the SADF)
Conventional wisdom has it that the whites would have lost - I am not so sure. I was part of this machine for many years, and there were some really tough people out there, both physically and mentally. And bear in mind that if the brown stuff had hit the rotating air impeller, all the factors regarding white disinclination to play would have dissapeared in a blink - survival would have been the only issue.
I believe that it would have been fought out to the point where all that was left was stones to throw at each other. International intervention against a demonstrated nuclear power - and they probably would have used them if the Cubans intervened(And one that would have shown itself to be clinically insane by the time an intervention could have started)? I dont think so ......
BTW I have never believed that there were only six nukes - those were proof of concepts and prototypes