What about a probability, that there was a mountain wave and extreme downdraft from thunderstorm or microburst activity that made them lose a lot of hight in an instant? or maybe the control-surfaces got damaged by extreame turbulence and became uncontrollable. Could that be possible ?
Just a month ago, I had a personal conversation with this very captain himself about exactly this subject because I wanted to learn something - he explained the dangers of this particular region and the problems of flying into Goma in IMC (which is basically next door) in great detail. This man had over a thousand of hours on type and has flown in the region and knew what it takes to fly there. It would surprise me, if they really did go below MSA volountarily, if not clear of terrain. Rudi was just about to upgrade to captain himself and has also had his share of Congo experience before this tragedy struck.
There used to be an GNSS Approach published in the Jepp. 2006 Don't know if it has been withdrawn. However ZS aircraft and crew as far as I know, have to be certified to use thease approaches and GPS is not permitted as a sole reference for navigation.