Weather on Mull
Dalek
If the observer was an estimated 100 metres, lets be generous and say 200 metres, from the crash, then the crash was 100 (200) metres from the observer who was in thick fog. A moot point as to how the aircraft could have been VMC, 1000metres viz in all directions. However, I accept your point that, once again, there is no absolute certainty.
SFFP
By your definition, if an aircraft crashed in poor viz on the threshhold of a runway, then ATC are not at the crash site.
Looking a bit wider, prior to this particular case and thinking back to 35 years worth of reading accident reports, I'm hard pushed to think of a case in which a number of witnesses stated that a hill feature was in fog and an aircraft hit that feature, 810 ft up, yet doubt remained about the weather conditions on that feature.