Was the Namib Desert a natural formation created millions of years ago, as science suggests, or was it, like the Sahara Desert, created by extraterrestrial dumping of sand over this vast area? If the latter, did this occur when the Earth was in its present orbit, or when it was orbiting the Sun’s companion star that is now a brown dwarf?

Your intuition once again is serving you quite nicely. This was a similar incursion by the Anunnaki extraterrestrials to punish people who were living in that region of the planet when the Earth was orbiting its companion star. This was the first cohort of divine humans that were created, but ultimately annihilated by the extraterrestrials. The Namib Desert is one leftover consequence of their manipulation and subjugation of that first human group. It was done to punish and to render useless a former area of human habitation and was highly effective in contributing to the annihilation that took place.