This sounds good except for when Insurance Companies get so good at determining who exactly is likely to be "medically unlucky" that those people are effectively prevented from getting any healthcare at all.

This is a very real problem for entrepreneurs who want to buy their own healthcare, and also for very small companies. I know of one company that was basically prevented from hiring an extremely talented individual because that individual was flagged as a high risk to be "medically unlucky", and if she joined the company than everyone in the company would be ineligible for healthcare - in this particular instance if you're less than five people than you can't buy a group policy with one of those five being a high-risk individual.

So these high risk individuals are either able to join a large company where they can be covered in a large group plan (better hope you're a skilled worker, since very low-end employers have incentives to hire part-timers or contractors to avoid paying benefits), or they have no coverage and go to the emergency room when catastrophic problems emerge, at great cost to the taxpayers who pay for those emergency rooms which (for quite good moral reasons) don't turn away patients who can't afford visits.

Maybe this is one of the things you grouped under "various important details", but I think it's more than a detail - denying coverage to risky people is going to be an inherent consequence of introducing profit motives into health insurance.

I don't necessarily think socialized medicine is the answer, given the inefficiencies it introduces. The best middle ground might be greater regulation of insurance companies to prohibit them from denying coverage to sick individuals. But given the risks on each side, I would frankly much rather err in the direction of too much socialization.

the high risk individual could have already had individual coverage, and kept it, and been paid a higher salary to compensate for less benefits.