would not an engine under power pick up debris, rocks etc during the crash sequence with possibly similar effects?
Yes it would. The damage suggests that the engine did produce power at impact.
On a Namibian pilot's forum quoted by AVH it has been reported that the weather was clear at the time of the accident, the line of T/S only moved in later in the afternoon hampering SAR. It appears that weather did not play into this, something seems to have gone fundamentally wrong mid-cruise.