This is pretty late in the conversation but I wanted to chime in with a couple of things as there are so many things that don't make sense to me, thoughts I have about this, and how it could have occured or been prevented.
Per Webster... master: one with control or authority over another or others ; a male teacher or tutor
Sooo, being a relatively new diver myself it was my impression that the "divemaster" was in control and that they determined the dive and took responsibility for it. Of the few dives I have under my belt, that has definitely been the case. They chose the location based on conditions and input, they chose the direction and profile, they setup the equipment...yada, yada, yada
Now, that being said, that doesn't mean I didn't/don't double check everything and exercise my own judgement. What may be perfectly within your comfort level doesn't necessarily mean that it's with in mine and vice versa even with the same experience.
However, it's always surprised me just how willing people are to be led and trust in others (especially with a title such as "divemaster"). Their unwillingness to step up in a questionable situation and be heard or stray from the group. In a crisis situation where someone needs to take control....forget about it, even less people respond. Then add in an "alleged" ego from someone whom I'd assume was most if not everybodies elder.....
I think all of this contributed to an ending with traggic results. Although I must say, if what the original OP states is true, they did do this to some extent by refusing to go to such a depth and taking responsibility for the fiance, in my mind this would obsolve them from the other group and no responsiblity would lay with them. This dive had 2 divemasters of sorts and the fact that the divemaster for the 100' club looked to the other for their missing diver shows that they were not clear as to the division of the groups. Poor dive planning to say the least!
But, that being said if this guy hit 346' and made it back to the surface in 22 minutes? He was either determined, heavily overweighted, and or narc'd badly somewhere around 100' (if he intended to do the dive as planned), but even still to get down there and still make a conscious decision to presumably make an emergency bouyant ascent?????? If he was that determined, he was going to do it and I don't know that it's the divemasters responsibility to reign somebody like that in and risk their safety as well as that of the group.
One other thing....coming out of class we had covered nitrogen narcosis but I didn't have a good understanding of it until I started reading this board. I certainly had no idea an experienced diver could have a bad experience with it 90 some feet. With the rec limit being 130' and being told that the dive tables were conservative, I think it can easily give people a false sense of security.
Somebody also made the comment about the lack of instruction and online courses being part of the problem. Like has been mentioned, those experiences vary. While I have had issues with DM's or instructors, my personal certification experience was very good and I did it online (through PADI) and then my instructor of the confined and OW dives was great. However, while I retained a lot of it, it really solidified a lot of it teaching/explaining it to others that took the class later.
But then again, like someone said, you don't know what you don't know until you know!
Sooo, I know I just managed to ramble a lot, but to the OP, I'm very sorry for your experience! BUT, looking for someone to blame will not solve the issue, finding the root cause and sharing it will not change the past but may change the future and I hope you can atleast get some comfort in that.
Take care,
Steve