Nothing wrong with what your saying about spacial disorientation, however, we don't know if this is the case yet. personally I'm prepared to pay the benefit of the doubt at this point. Given he only just landed, unless there is a witness that can say I could hear it takeoff but I couldn't see it because of the fog, then we have to accept that he didn't, in which case carby icing would be my next choice in those conditions.
If he didn't notice the power degrade on takeoff, it would get worse he could have just simply descended into the fog in the south west corner, the outcome would be exactly the same. The evidence would melt away and we would never know.
Just thought I would offer an alternate sequence of events at this point