For Pretoria to Windhoek I usually go west on the N4 via Rustenburg to the Bots border. It the becomes the A2, go all the way along there via Kang and then cross into Namibia where the A2 becomes the B6. You just follow that all the way via Gobabis to Windhoek. It can be done in 2 days if you are used to long drives.
Does your amount of time available included getting to and from Windhoek from Pretoria...if so then I would lean more towards the 14 days rather than the 10, as you will be limiting the amount you will be able to see.
For Etosha it is good to camp inside the park as you have the floodlit waterholes.
Namutoni campsite is the nicest for camping as it has grassy campsites and shady spots, but generally I do find the game viewing to be better around Halali and Okaukuejo in July.
I usually push through as far as Kang and then overnight there.
If you don't want the next night to be in Windhoek you can just go a bit further north and camp at Dusternbrook Guest Farm, lots to do there including leopard and cheetah viewing, rhino tracking and horseriding.
From there you just have a few hours to Etosha.
Waterberg Plateau campsite is nice for the way back down. Lots do do there as well.
Gemma Dry - Owner - Discover Namibia Safaris