SASless,
As you rightly point out we simply don't know and wont know until the wreck is found. Every theory postulated including mine is a guess however well informed. Hindsight is also a wonderful thing, but...
If I were the skipper of that boat, knowing I had taken in two tons of oggin through a dodgy snorkel I would not have dived it. I would have stayed on the surface and issued an SOS. Something catastrophic happened very quickly.
My thoughts are with them all.