The logical conclusion, then, is that the most cost-effective structure is the maximal one (a single risk pool including the entire population, a.k.a. universal coverage).

And the evidence from countries that have implemented this supports that conclusion!

Not necessarily, people who are unemployed tend to get sick more often with the most common cause being substance abuse. A company with workers that are drug tested is likely to spend less money on substance abuse programs than a group of random individuals.

I think you need to back up claims like that with a source. Are companies themselves spending money on substance abuse programs for their employees? Are substance abuse problems the biggest factor in health insurance burden?

No insurance provider has only one customer. They always pool risk across all of their customers. A company signing up lots of their employees doesn't increase the size of the pool -- it just adds an intermediary.

The reason often cited for employer-based coverage is that it avoids some self-selection problems, but that's a different and complex topic.

Self-selection is certainly a fundamental economic problem of health insurance. Large employers can make their own deals with insurance companies, and small employers can participate in small group health insurance plans (which effectively combine all the small businesses in a region/state into one group).

>> A company signing up lots of their employees doesn't increase the size of the pool -- it just adds an intermediary.

I'm not exactly sure what you mean by that. For large group insurance plans, rates are determined by the risk (calculated by the insurance provider, with certain legal restrictions) of your individual employees. If you have 100 employees and 10 of them get a chronic illness, your rates will go up. Similarly, an employer with older employees will have higher rates than an employer with younger employees. The pricing is per group (employer), not based on the entire pool of the insurance provider's customers.

For very large employers, I believe it's common to self-insure, i.e. make a deal with an insurance provider to provide care to your employees at the rates negotiated between the insurance provider and health care providers, but the employer just pays out the total cost of delivered care rather than premiums. Then you get into stop-loss insurance and other interesting complications.

That's what insurance is, no matter how you get it.