Lomcevak,
Most accidents of this nature are traced back to a series of unfortunate coincidences; the holes in the cheese.
The point you make is the right one though, and one I made earlier. Everyone seems to agree that rpm and hence thrust were reduced during the climb: either the pilot commanded it, or he didnt.
For my part, it seems inconceivable that he would have commanded it, hence I am thinking about whether it was another in a string of instances of an Avon 122 uncommanded thrust reduction.