FSB
Your are right, none of us know why this poor guy had the misfortune of crashing, and you are also correct to state we will not know the true cause until the AAIB get done,
There have been several posts that contained rather dismal Metars that allude to weather potentially playing a part, you yourself pointed out that conditions had been "vile" prior to the crash but go on to state the weather "cleared" and "It was as NOT foggy and it was NOT raining" and observed conditions being cloud at 400, Vis 3miles, wind up to 15k. The implication, if I am reading your post correctly, is the weather went from very bad to acceptable. It didn't, cloud at 400, vis 3sm and wind of 15kt are challenging conditions for a VFR pilot to fly or land in regardless of the absence of rain or fog. I feel obligated to point this out not to further speculate as to the causes of this crash but to hopefully avoid the next one if some poor novice reads your post and incorrectly interprets the conditions you describe as clear flying weather.