Some of you have contributed very insightful observations. They made me conclude the following (my pure conjecture):
1) Instructor knew full well where he was - he was teaching courses in this very area, I bet you he could get out of there anytime in any visibility, with no line or compass (it is a tube).
2) The autopsies did not reveal any trauma - there was no 'fighting'.
3) If you look at the YT video I posted showing the wooden frame area (the 1st video), you will see that there is an air pocket just before the wooden frame. I think this is where Student 1 and Instructor decided to 'breathe' and that's why their helmets were found there (it is easier to stick your face to the narrow air pocket without a helmet).
4) Using the air pocket could have been a way of beating the total silt out in order to communicate or check compass/map etc, as well as breathing the atmosphere that sustains life.
5) Student 2 took an oxygen tank with him through the stone dam during the rescue attempt to purge the air pocket, which due to breathing would contain more and more CO2. If purged with air, the O2 concentration would always be below 21%. With oxygen, the O2 concentration in the breathable pocket was being enriched to levels supporting life.
6) Student 2 left stages before the stone dam as he knew the team would need them to surface and do deco. It was impractical for him to squeeze with all four through the stone dam (the videos I posted contain sequences of divers squeezing through a stone dam before they reach the wooden frame - that could be the stone dam in question, it is not too wide).
7) Student 2 laid a line from the wooden log to the T area to prepare evacuation or show the way for the rescue team.
8) I believe Student 2 honestly believed the rescue party is coming very soon - there must have been discussions on the surface between the Observer and Student 2. Note, that the Observer did go into the water again after Student 2 went to look for missing divers. The Observer did not go past the dam though.
9) The Instructor did not want to leave the panicked student alone and stayed with him hoping for the rescue to arrive - he knew there are 5 divers on the surface who know they are in trouble, it would not have been totally irrational thinking.
10) What caused the initial bolting to the air pocket of Student 1 and subsequently the Instructor is unknown, probably panicking. Note that if you remove gear in zero vis there is zero chance you can find and put it back even if someone brings you a tank with air, which in that scenario were not even full.
I think we are looking here at two heroic efforts of Instructor and Student 2 to rescue Student 1. Unfortunately, they ran out of air as they did not have any fresh tanks at the surface and the rescue party never came.
