OP, the fact that you admit you were a danger to yourself translates that you were a danger to everyone around you in the water. That you do not realize this but are quick to and feel justified in criticizing another diver is more the real problem/danger.
Moving your arms while diving may not be an example of good techinique but it is hardly unsafe....the ocean (lake, pond, etang, etc) is a big place with lots of space for quite a number of divers, If you can't space yourself safely from another diver who's techinique is a bit erratic then you should stop focusing on the erratic diver and focus more on your own positioning in the water. Also, if visibility is more than a few meters then there is little excuse for divers, especially with experience, to be bumping into each other and kicking each other...in a low vis environment or a night dive where it may be more imperative to stay shoulder to shoulder I can see this, but during a dive with 2-3 meters or more of visibility being arms length apart is close enough for communication and safety.
Also, you remarked that this diver in question would be a danger in a swim through....it sounds from everything you describe that there were other divers that lacked experience in general among the divers in your group...particularly the 13yo you reference....perhaps your excursion group should not have been anywhere near a swim through....and if the decision was beyond your control, you could have elected to pass through before the erratic diver or after them negating the so called safety issues you talk of.
A regulator being inadvertently pulled/knocked from someones mouth is not necessarily an emergency, it is something that is trained for during basic open water cert courses. In the scenario you describe the unlikely event would be the diver using their arms would snag a hose and yank the regulator out of someoneelse's mouth...this is unlikely to cause injury, the diver affected should have enough spatial awareness to keep clear of the guy, and if it was to unfold where the reg got yanked out it is as simple as grabbing the octo/safe second and plopping that in the mouth and then recover the primary second stage.
Having a mask knocked off, again is not an emergency, and is a skill specifically trained in every basic open water course. It could be problematic if the mask is lost, but again all that means is some discomfort and an aborted dive, rarely does it involve injury, and in the case where one is better off ascending the anchor or other line for safety, then the diver with the lost mask can be escorted back to the line by their buddy where they will then abort the dive...again no danger, just some discomfort...and great learning experience.
From everything you have brought up for discussion, the most legitimate thing that should have raised any concern is that there was nobody specifically paired with this guy to look out for him should a serious issue have occurred. Buddy diving is the standard taught in every basic open water course that I am aware of. To me, if it is truly the case, that this guy was not paired with anyone and just gommed onto the group at large, then that is an egregious error on behalf of the dive-op/DM, while not ideal, they could have at least tripled up one of the buddy teams if there were any acknowledged buddy teams at all. Everything else you have mentioned are really non-issues and can be mitigated by persons being aware of what is going on around them which is an individual responsibility.
There is no diver's code like the skier's code where one person has the right of way in certain scenarios. Even with boats where there is right of way/rules of the road, the imperative is to maintain awareness and avoid collision at all cost if possible...if it is possible to avoid collision but action is not taken, the rules of the road for open and inland waterways is that both vessels are at fault (or course there are specific exceptions but the struck vessel must prove that they were not capable of maneuvering).
The fact is that none of the issues you present actually happened, and if they did occur would only present as truly dangerous in the most extreme of circumstances, makes your concern comes across as you not wanting to share the open water with someone less skilled than you. True/not true...only you can judge that.
Among your mistakes, was you interpreting the other divers experience based on the fact that they had well worn/vintage gear. People buy and sell old gear all the time, it means nothing with regard to experience. It could very well have been this guy's 1st post certification dive(s)...nothing wrong with that, nothing wrong with rapid air consumption in this scenario, and nothing wrong with excessive arm movement and kicking either. It is not only not dangerous but to be expected of a beginner diver....All those things only get better with time/experience under the water.
The story you posted seems to reflect more about you than the other diver.
Plan your dive, dive your plan, endeavor to leave the water a better diver than you entered the water, and help others do the same. Put your ego in your BC pocket, or better yet leave it at home...despite the vast amount of space in the open waters of the world for divers to explore and enjoy they seem awfully crowded when people bring their egos along for a dive.
-Z
