That mist was forming as the prevailing air current rose up the Mull and therefore was localised - common conditions there - a short way out to sea conditions were fine for VFR - they just needed to judge their distance off the Mull.
This was a regular flight plan - a milk run - there's no way such conditions would have mandated IMC in the vicinity of the Mull spoiling so many such flights.
It boils down to how they were judging - or measuring - their distance off before their planned close in turn.
All the conjecture about possible mechanical/control problems (that leave no trace) is puzzling to me - the a/c needed to have been pushing along at its high cruise speed in a normal attitude without disturbance to have got to where it did when the final evasive manouvre was executed - at which point it seemed to have been very controlable.
The Americans were so close by that they were lucky not to have been hit by pieces of wreckage ... do any of them have any comments?