Sadly, the MSA is a fact, it can be read off the chart. Equally the altitude of impact is a fact and has been measured at the value stated. The diffence between them is a simple calculation.
The aircraft was in contact with Bukavu and made no indication of any distress. There is in fact no indication of any kind of aircraft failure, in any of the evidence.
As difficult as this may be you may have to accept that the overwhelming body of evidence points to Controlled Flight into Terrain. It is unlikely that any more is going to be done to discover anymore, the DRC just isn't that kind of place.
May your son rest in peace.