These are good and interesting questions, but the problem is that research in this area is still relatively new. Which isn't to say that nobody wondered about the Sahara or Roman north Africa before, but by and large our archaeological information from the Sahara itself is largely from the past ten or so years. This means that a lot of the questions you are asking are still being worked out.

Now, that being said, it appears that it is better to think of the southern border not as a border or a fixed line, but more as a broad and difficult to define "frontier zone". There is a wonderful scholarly definition of "frontier" that I can't remember, but broadly speaking a frontier is a zone of fluidity in which power relations are shifting and uncertain. The Romans built forts within the Sahara itself, but this does not mean they controlled a region as a whole--in fact, the general consensus is that Roman military presence along the southern border was more for taxing and monitoring trade than imposing direct control. Personally, I think this conclusion can be pushed somewhat too far, but by and large the Romans secured their southern border more through diplomacy than with force.

My main source is Andrew Wilson's "Saharan trade in the Roman period: short-, medium- and long-distance trade networks" in Azania (Nov. 2012). A lot of this is wrapped up in discussion of the Geramantes, which are given a good discussion in Katia Schorle's article in Sahara.