How do you know it's going to be a cold day on safari?
When your PH shows up for breakfast wearing 2 pairs of shorts.
I brought a couple pair of Carhartt long pants my first time, and a couple pair of shorts. I discovered that the long pants don't offer a great deal of protection from thorns, and besides which they're pretty noisy rubbing against thorns or getting caught in them. And I still got scratched up.
Tsetses are generally not an issue above about 1200 ft elevation, which you should be well above in the eastern cape. Mosquitoes are generally not an issue above about 6000 or 7000 feet of elevation.
I mostly wore my shorts. For the "chilly morning" problem, I resolved that by wearing some camo, lined rain pants while in the bakkie, then took them off when it was time to get out and start walking.
Every bakkie in camp had an untold number of tire patches from thorns and tree branches an roots and what-not. If tires can't resist thorns, long pants won't, either.
40º mornings sitting in my South Louisiana tree stand waiting on the white tails to show up is a very different proposition to a 40º morning at high elevation where there is no humidity. Still cold, but 40º in S Louisiana chills right to the bone and I can't take but a couple hours of that. At high elevation and no humidity, I'm perfectly comfortable in 40 - 45º weather in shorts and short-sleeve shirts once the Sun comes up.
Evenings around the braai are as chilly as the mornings. You'll want 1 pair of long pants for certain, and a medium weight jacket.