Cessna 340? I have some time in 340’s. Nice useful airplane, but if these folks were normal sized people, there is almost no chance this plane was not over gross, even with minimal fuel on board. Maybe not overweight, but doubtful. In a fogbound takeoff, you wouldn’t need an engine failure to have an immediate and serious problem, you’d just need an engine to “hiccup.”
My dad flew a couple of 340s and I wrenched on some. They had about 2k# useful load. Five 200 lb bodies and 150 gal fuel would be getting up there. Supposed to have been a day trip, so maybe the luggage was minimal.
But the temptation to climb up out of the fog pronto, with the possibility of the weight and balance too far aft (folks in those type of seating like to face forward, leaving the aft facing seats filled last.)
I wouldn’t be surprised if he simply pulled the yoke back too much too soon, in trying to leap out of the fog, and just spun it in.
I dont what engines he had on it, apparently some had more powerful ones than the originals, and some were retrofitted with turbine engines.