I agree that people should be responsible for their own safety, but beginners need to have their hand held.
If someone is certified, an instructor has determined that the diver is capable of planning and executing dives with a buddy within their experience and limits - no hand-holding required.
If hand-holding is required, it suggests that the diver is out of his or her comfort level and the diver should consider aborting or at least hire a DM or instructor for that purpose. It helps if the particular dives are worked up to gradually after achieving comfort in easier scenarios diving with a non-professional buddy.
If they confuse 3,000 psi with 300 psi, it's a communication problem that could result in a serious accident/death.
A diver should not still be at depth with 300 psi. The diver should have already informed the buddy first and the DM if necessary when they reached a pre-determined pressure and began their ascent by a predetermined pressure with the buddy - regardless of whether they could get the DM's attention.
Many of us on this board begin our ascents at around 1000 psi or more depending on the type of dive and ascent requirements.
As a new diver, I can tell you that during my first dives, I did EVERYTHING the DM tells me to. There is so much going on all at once, that you can't rely upon yourself for making all the right decisions.
If there is so much going on that a diver can't rely upon him or herself to make the right decisions, it again suggests that the diver is diving beyond their comfort level. All the basics should be just that - basic. If one is struggling with the basics, then it puts you at increased risk if things go wrong, since that would become really overwhelming.
I spent my first 38 dives in various training courses/shop dives with instructors and/or DM's nearby because I liked the comfort of having a leader around just in case. Yes, they were SCUBA Gods to me until I started noticing mistakes and hearing about and reading about accidents and incidents, and I realized they were human and I had better be responsible for myself. I did my 39th dive in dark, silty, low viz water with two buddies that I knew already - conditions we were used to, but a new dive site for us. We had a couple of minor issues, but we handled them well, and my confidence suddenly went up. From there, I just kept planning and executing dives with similar buddies that I trusted, quite a few of them instructor friends
and building up to the depth of dives that I was actually trained to. The confidence and self-reliance will come as you do more dives, take more responsibility for them, handle issues, become more relaxed and proficient, and gain more situational awareness. I found that the rescue course really helped in showing me that I was capable in self-rescue as well as in rescuing others and encouraged me to be more self-reliant and aware of situations. I think all of those things could help many new divers.