Namibia is my favorite country on earth, and I'm really pleased for you that you'll be going there.

Personally, I don't find Walvis Bay or Swakopmund particularly interesting, however others may differ. There's some great quad biking to be had in Swakupmund, though, and I'd go back just for that. Another popular attraction there is dune surfing.

Sossusvlei should definitely be on anyone's "don't miss" list of places to visit, and no matter how hot it is when you get to Big Mama, DO make sure you go into Dead Vlei. Most safaris leave early in the morning when it's cool, so that you're back at your camp by the hot afternoon hours.

All my Namibian safaris have been through Wilderness Safaris (the link is to their Namibian properties), but they can be very expensive, and knowing nothing about your circumstances, they might be out of your price range.

When my parter and I were in Sossusvlei in 2010, we stayed at Little Kulala, though there is another Wilderness camp nearby called the Ongava Tented Camp. Little Ongava is wonderful - the beds in the bedrooms face giant picture windows that show the dunes' changing colors as the days progress. There are also other options in the area, (including Sossusvlei Lodge, a stone's throw from Sesriem Canyon) that can fit tighter budgets than the Wilderness Safari camps. None of the accommodations will allow you to walk to the major dunes in Sossusvlei - the dunes are actually inside of a national park, which is gated at night. Almost all of the camps offer trips to the dunes, and depending on where you stay, the time to get to Big Mama can be anywhere from 60 to 90 minutes, mostly on a paved road once inside the park.

One other place to consider visiting in Namibia is the Serra Cafema camp (see above link), located on the Kunene river, which borders Angola to the north. This is my favorite camp anywhere, and it brings new meaning to the word "remote." This is probably my favorite place on earth, come to think of it.

Anyway, back to your question, your itinerary sounds like it would work well for you, and I'd be keen to see what others think/suggest.