You should see plenty of wildlife in Botswana in a lush landscape (although some trees will be leafless in winter), so no need to visit similar wildlife areas in Nambibia. The Skeleton Coast and Sossusvlei will be dramatically different from Botswana. A nice contrast.

Yes, allowing to full days at Sossusvlei is a good idea. Although a full day is sufficient to visit the vlei itself as well as Sesriem Canyon, you never know when the mist will roll in. An extra day increases your chances of clear skies.

By the way, don't forget to look up at night. There are few places on earth where the stars twinkle and sparkle as spectacularly as in the Namib Desert.

johan