1, I agree that solo diving is an activity that should only occur once the diver is an accomplished buddy diver. It's a refinement of skill, rather than an excuse not to possess skills.
2, I agree that appropriate training should be obtained. Many agencies (including PADI) now offer solo diver training - and they tend to be exceptionally valuable courses. The PADI version is called 'Self-Sufficient Diver' - in recognition that the training provided not only permits solo diving activities, but also an evolution of self-sufficiency for a diver who may otherwise operate in a buddy pair or team.
3, IMHO, divers who solo diving without appropriate qualification tend to fall into one of two categories;
(a) Cowboys, who put convenience before safety and don't have an integral ego-check to understand the relationship between experience/skill-set and risk. Typically defined through an 'acceptance of risk', rather than a 'mitigation of risk' mindset.
(b) ' Old-school' divers, who undertook more extensive diving training long before the 'fast-track' short courses of the modern era - who could otherwise be expected to possess a refined sense of personal responsibility and a high degree of experience. Whilst not possessing a 'qualification', an analysis of their experience and training history will normally illustrate that the required skill-set and mindset are in place.
4, As mentioned, mindset is critical. To solo dive prudently, the diver needs to be confident that they have mitigated all foreseeable risks. That confidence needs to be based on a foundation of honest self-awareness ("how good am I really?") and a comprehensive understanding of, and respect for, the risks they will face ("how prepared am I, when compared to the worst case scenario that I will face?")
5. The diver must have an honest belief, based on evidence of past-performance, that they can handle emergencies calmly and with control. Panic is the critical factor and will nullify any hypothetical planning or equipment carried by the diver. It's hard for an inexperienced diver to have an honest belief in their performance under stress, if they haven't had chance to experience stress and deal with incidents in the water. Role-playing/simulation in training can provide an insight into this, but there really is no substitute for 'hard experience'. If a diver hasn't had to deal with a given situation, then they shouldn't assume capability for that situation.
6. Never fall prey to the ego-trap that tries to tell you that you are competent, just because you never experienced an emergency. Diving is a safe activity and you won't generally face critical problems on a regular basis. You are not a safe or competent diver because nothing goes wrong - you are simply a lucky diver. Sooner or later, luck runs out. Doing X number of successful buddy/deep etc dives is not a criteria for deciding your competence under those conditions... or deciding your capability to progress into more demanding or higher-risk activities. Performance under 'best case scenarios' is not a determining factor for your ability to survive a 'worst case scenario'.
7. Solo diving places a critical importance on the issue of stress management. In a buddy team, there is always a 'back-up' brain - a person who may not be directly impacted by the emergency and, therefore, more likely react with calm and control to help a resolution. When alone, the stressed victim is also the person who has to resolve. Never underestimate how stress/panic can degrade your ability to resolve a problem safely.
8. Diving emergencies occur in-line with the principle of 'Murphy's Law'. It's easy to rationalize that a particular risk is unlikely (and therefore dismissable) - but the random occurrence of such events is what causes diving fatalities. Nobody died on scuba that predicted, or expected, the incident that killed them. If you haven't actively mitigated a risk, then you should dive in the assumption that it can happen and it can kill you. Use those assumptions to guide you when setting your own personal limitations. In that respect, don't be pre-occupied with the issue of depth - unless you possess gills, then having no access to air at 20cm is just as lethal as having no air at 20m.
9. Diving emergencies typically feature a 'chain' of events. That is how people die under seemingly innocuous circumstances. Learn to recognize when you are on that chain... and be prepared to break the chain - if nothing else, by simply aborting the dive as a prudent measure. Also be aware of your capability to deal with multiple issues simultaneously - it's easy to rationalize your capacity to deal with a single incident/stressor - but reality proves that such things rarely happen in singularity. A single incident/failure/stressor normally results in a 'near-miss'... you survive. Multiple simultaneous incidents/failures/stressors have an exponential capacity for decreasing your chance of survival.
10. Lack of skill, knowledge and capability drastically increase the risk of multiple simultaneous incidents/failures/stressors. This is the scenario where task-loading, stress and/or inappropriate response dealing with a primary issue can actually lead to the evolution of further subsequent issues. Be aware of how this 'chain' can work and ensure honest confidence in your ability to severe the chain through appropriate and timely response, with an emphasis on not creating further issues.
11. Be pessimistic in your dive planning, risk assessment and risk mitigation measures. Never assume that you can simply 'avoid' a problem. Never assume that a problem will arise. Be prepared, mentally, skillfully and with appropriate equipment, to deal with a problem. "Avoiding entanglements" is not a risk mitigation... "being prepared and equipped to deal with entanglements" is.