Not sure there's too much to be learned from the OP's post.
You contradict yourself, because you continue to describe some very meaningful and educational conclusions....
...dive computers vary significantly w.r.t. NDL calculations
Lesson #1: Dive computer algorithms vary, so a diver should not assume that buddies/team mates/group divers are being advised of the same NDL as them.
Lesson #2: Time-to-NDL (
remaining bottom time) should be frequently communicated and confirmed, as standard, between buddies/team/group.
....distracted by the two divers lagging behind which would have been annoying.
Lesson #3: Bad divers can cause further task-loading for the buddies/team/group. This can reduce situational awareness - increasing the risk of mistakes and accidents.
Lesson #4: When diving with unknown or less predictable divers, a deterioration in communication and team skills should be predicted. This should necessitate a raised situational awareness.
...concerned about the other two divers.
Lesson #5: A problem arising with the buddy/team/group frequently cause elevated stress and anxiety in the observing diver.
Lesson #6: Problems without pre-considered solutions cause increased stress and anxiety.
Lesson #7: Deviations from expectations (
whether or not assumed or explicitly communicated in dive planning) can cause elevated stress and anxiety in a diver.
made sure his dive plan was sound.....
Lesson #8: Diving planning must be conducted as a group, if diving in a group (
a buddy pair if diving as such, or a team if diving as a team). Dive planning must be comprehensive and fully inclusive of all divers undertaking that planned dive.
Lesson #9: Dive planning must not assume shared expectations or identical protocols for each diver. One purpose of dive planning and preparation is to ensure a mutually understood approach and eliminate false assumptions. The less familiar/intimate the participants, the more fundamental and detailed the dive plan should become.
Lesson #10: Some divers (a minority) will demonstrate inherent disregard for the planned dive and other basic safe diving principles.
Lesson #11: There is nothing a diver can do to enforce compliance amongst divers who are prepared to disregard planning or agreed buddy/team/group safety parameters.
Lesson #12: Every diver should have a self-reliant plan to preserve their own safety in the event that their buddy/team/group take actions that jeopardize that safety. I suggest this contingency plan should be
very similar in protocol to that used when separated from the buddy/team/group.
and when the time arrived to surface, signal his buddy (i.e., guide) and surface
Lesson #13: Dive guides/divemasters may have conflicting priorities between a designated buddy and the wider group.
Lesson #14: Some divers, including guides/divemasters, may not respect the prudent rule of "
any diver had the right to abort the dive at any time, for any reason". It is wise not to assume they will respect that - and contingency plan for your own safety as a precaution against it.
Lesson #15: Communicate and confirm that priority, and resultant contingency plans, in expectation of that.
.. make sure to learn to read the dive computer properly. Underwater is not the time to guess what "10" means.
Lesson #16: Train and familiarize yourself with every piece of equipment that you use.
Lesson #17: Familiarization includes all functions of that equipment, even those that are for contingencies/emergencies only and, thus, rarely used.
Lesson #18: Planning and preparation for emergencies (including unplanned deco) must happen before the event, not during it.