I agree with many of your points. The one sticking point for me though in the approach you advocate is that medicine is not like having a kitchen remodel. You cannot realistically get a number of estimates and make an informed decision about which offers the best deal. Sure, you can shop around for an elective MRI and have it done at the least expensive location (MRIs are MRIs, essentially commodity items and if you wanted a different reading you could pay extra for that, zero reason for one setting to charge $500 and another to charge $5000). But you cannot shop around for emergency surgery and unless you live in a major metropolitan area, your ability to find enough physicians and hospitals in true competition with each other is very limited. It isn't really a free market out there. As such, there is a need for the sort of "collective bargaining" on your behalf that an insurer can do. That $5000 charge for a colonoscopy far far exceeds what it actually costs. If your insurer negotiated with the hospital to pay only $2500, you can be quite sure that the hospital still made a goodly profit. However, if you had, as an individual come to the hospital they would have more than likely stonewalled you and charged $5000 or maybe $4500. If they charged less they would run into trouble with their big insurance companies who have deals that say essentially that they will pay half of what the regular charge is. If the hospital let you pay $4000, the insurance company would want to pay only $2000 and that might squeeze the hospital profits.
Medicine is not a free market. We, as consumers, need the protection in that marketplace that only comes with either regulation or negotiating power. The regulatory strategy just ends up creating a static system for the doctors and hospitals to game. The large insurers on the other hand can be flexible but still wield considerable market clout and therefore benefit us. The down side of the insurance strategy is overhead. If you have one big insurer (eg Medicare) then the system supports one chunk of overhead/infrastructure to make things work. If there are 500 insurance companies, then the system needs to support much more overhead and that drives costs up as well as reducing the clout of each individual insurance company in the marketplace. That is why single payer is inherently more efficient.