I have no dives under my belt except my cert dives so take my comments as what you paid for them.
Seems to me like you have two choices. First, you can let the "hot shots" be themselves or you can find a different club or dive buddy.
Everyone has different limits and capabilities. Many, probably most, of us test our own limits until we make a mistake. Going deeper may not be a problem at all for your first diver and so far he's been successful. Success emboldens us, especially those of us who are already confident (or perhaps overconfident). I wouldn't do it, but I'm older and less likely to push my limits than I was at 20 years old. He's clearly capable of doing these dives if nothing goes wrong. He may be capable of doing them if something does go wrong. The only way of knowing is to do the dives and have something go wrong. Until then, it's all speculation either way.
Your second situation/person doesn't seem like a problem at all to me. Neither diver seems to have been "solo" and deliberately leaving anyone behind. You said vis was crap and that two divers continued while four remained. You said they surfaced only a few minutes later than the rest of you but were a distance away. I'd suggest they figured they were still "with buddy" and thought you'd regroup but then when you didn't they surfaced. Distance is understandable, especially if they took a minute to realize you weren't still all together and then took some time to look around for you.
You have the option of talking to these people, but taking only from what you've said, your attitude is just as problematic as theirs is. Your post implies some envy/jealousy, whether intentional or not, that skews your perception of their motivations for diving and their level of safety consciousness. You sound like you're upset because you can't change them to your way of thinking.
There's nothing inherently unsafe about following a dive computer over charts. We weren't even taught charts in my class except to explain "hey this is a chart, depth is here, time is here, you'll never use them everyone uses computers nowadays". My PADI class materials came with a little eDRP computer and absolutely no charts whatsoever. There is similarly nothing wrong with using an air2 over an octopus and a lot of divers apparently like them. As for long hose versus short hose, again, that's personal preference and it's very rare to change someone's opinion until they're directly in a situation to see the benefits of both side by side and do a comparison.
People tend to like their own gear, especially if they spent a lot of money for it and they're still relatively new to it and the activity. This is the same for all activities. It takes time and experience to learn what's good and bad about each piece of gear.
You're fighting the battle of inexperience on top of the hubris of youth (assuming they're all university age people) so if they're unwilling, or you're unwilling, to have discussions about pros and cons of different dive objectives, and safety options, find different people with whom to dive.
I can't say what's a safe number of divers under one flag, because I simply don't have the experience to know that. I can say, bringing it up as a group, before your dives, and making clear plans is the best way to fix that issue. You can always provide a second flag if you feel you need more. From what it sounds like, the dives have generally gone according to what I would see as safe, when you split up you all surfaced, even if at separate times and locations (which seems reasonable to me) and you've had at least some sort of discussions about safety and gear. It takes time to sort through people you want to continue doing an activity with and those you don't. You're doing that, but from my perspective as a very active person in other "life & death" sports who's new to diving, your concerns are out of proportion with the situations you've described.
