i do agree with above posters. swakopmund is nice little town, and the location on the sea with the desert "just behind" the town is nice. but unless you are into adventure activities, 3 nights are too much in my opinion. if i had to "cut down on time" anywhere on your itinerary, that would be my first choice.
sossuslvei is really beautiful and deserves more than one night, especially considering that you will drive there from windhoek and arrive in the afternoon. 2 nights would probably also be "enough", but with 3 nights you have the "luxury" of maybe drive to the park on the first day, and on the second day, spoil yourself on book at hot air balloon flight over the namib naukluft park. when choosing accommodation, look at how far away from the gate as sesriem the lodge / camp is. you want to be at the gate when it opens (at sunrise), and the further away you are from the gate, the earlier you will have too get up (and the longer you will have to drive in the dark).
with 4 nights in etosha, i would probably stay 2 nights in a restcamp and 2 nights in a luxury lodge just outside the park. there is no real disadvantage in staying on a campground outside of etosha. but given the choice, i would prefer camping inside the park. the restcamps all have a waterhole just infront of them, and its great to sit infront of them and see and enjoy wildlife. of course you can do that whilst visiting during the day, but if you stay in the restcamp, you can also walk there before having breakfast, or before going to bed etc. (when all the day visitors are gone).
with 1-2 "extra nights" (if you spend 1-2 nights less in swakopmund, or a night less at sossusvlei), my first choice would be twyfelfontein / damaraland btw swakopmund and etosha. one night is a bit short, but better than nothing, two nights would be better.
cheers, nathalie