Fanaticism is not an answer to tanks, planes, nerve gas and machine guns I am afraid.
Don't forget that the African side, i.e. Angola, Mozambique, Zambia and the ZPLA, will also have those things, except maybe the nerve gas part, although they may also resort to that as retaliation for NRA chemical attacks. And the Soviets will be more than happy to sell more. Drew hasn't been very clear on the frontlines in Botswana, but it would be very easy for the NRA to occupy most of the country, thereby reducing the Botswanan Army to a similar position the Belgian Army was in WWI, relying on their allies, or going to the bush to become guerrillas. Though considering that 70% of the country consists of the Kalahari desert, control of Botswana would really be dependant on control of the central nodes (population centres and highways) rather than the entire area.
The fighting performance of most rebels movements whether in Angola, Mozambique and Rhodesia was average OTL since they all lacked training and discipline. It is also worth noting that in none of these cases did South Africa fully involve itself in these conflicts. While South Africa did got a bloody nose in Angola during the eighties, it was only because of massive direct Cuban help to the rebels.
Point taken. Yet, the Portuguese are no longer in Angola and Mozambique and haven't been there for almost 40 years, yes? And the Ian Smith regime was unable to keep the blacks down forever. Yes, the Carnation Revolution played a large factor in the ending of the Portuguese colonial wars. However, a large factor behind the Revolution were those very same, endless wars that became a growing drain on the economy and more and more unpopular among the populace. Also in Rhodesia the war eventually became too costly, particularly with the oil shocks and rising White emigration from Rhodesia (which should be even more ITTL as Julius pointed out earlier). In both cases military force alone was unable to win the war for the Portuguese and White Rhodesians, respectively. The rebels didn't have to be great, they just had to be good enough. Or in other words, win the war by not losing.
It would be the same against South Africa. It is in a war it cannot militarily win unless it is ready to physically occupy the countries it is at war with. And even then it wouldn't be enough, as you would then have a whíte population of 5.5 million trying to dominate a hostile or potentially hostile (depending on the region) non-white population of 61.5 million, with the white militaries stretched thin. This is something that simply cannot be achieved without resorting to Draka-level atrocities.
The whites while fanatical are also not stupid and you can bet that in the case of Rhodesia units like the African rifles will still be operational. So will the Selous Scouts too and these guys were not pushovers at all, they routinely killed hundreds of insurgents in search and destroy raids.
The blacks certainly aren't stupid either, and they can adapt and learn from their mistakes. All those Soviet and East Block advisors will certainly help in that regard. It'll take a few years but they'll eventually get there. Meanwhile, the Selous Scouts and African Rifles will still be operational but for how long? Even they are not Rambos, and they will also take casualties. Only their losses will not be as replaceable as the ZPLA's and their allies, and the manpool will further shrink with white emigration from the worsening hellhole that is TTL's Rhodesia.
South Africa might be spending more money on internal security, but they will find some willing auxilliaries in the bantoustans who will be willing to "betray their race" for some amount of political power.
Now if the Soviets decide to help the rebels directly, things might change then but they will have other fishes to fry in China very soon.
No, they'll supply advisors and weapons instead, with other East Block countries supplying potential manpower (like the Cubans).
S. Africa may very well get some auxilliaries, but they will also face increased unrest from the black majority, particularly after Mandela's death, which will convince a lot of moderates that a negotiated solution is impossible and violence is the only alternative. Consequently, Umkhonto we Sizwe may grow from an urban guerrilla force into a general guerrilla army, spreading its tentacles all over S. Africa, from the Bantustans to the black slums and townships, forcing the S. African military to divert more and more men and equipment from the front, leaving the Rhodesians ever more in the lurch.