A single payer system has more support than you give it credit for but given the insurance opposition you may be right as to its political infeasibility. I certainly don't know.
However, your mixed system is probably the worst of the 3 systems because of the excess costs of private administration of even crappy plans. If you are looking for a "pragmatic" solution:
3a. Mixed system--expand Medicaid (or even better Medicare!) to everyone not covered by private insurance.
A few points about this mixed sytem. Its even easier and cheaper than Wojtek's to implement. If managed Medicaid could be limited someway it might even provide some adequate quality. There is some private insurance "crowding out" that pragmatist's would worry about (employer's choosing not to give their employee's insurance) but basically unless the economy went completely in the tank the switichover would likely be small (as the quality and options available in Medicaid would be less than medium wage workers would allow).
Now, the kicker in all this is that we could have had this in the U.S. While Clinton's plan was floundering on a few dozen votes, Fortney "Pete" Stark (D-CA) proposed such a plan. The media pundits said that this plan was the "compromise" plan that could pass--but Bill knew better (this decision not to "compromise" for a more progressive plan that more Republicans and Dems would support is the basis of the conspiracy theories floated after the Clinton plan failed).
Peace,
Jim
"Compromise used to mean that the middle of the road was a safe and reasonable place to be. Now it seems to mean acquiesence in getting run over there."
--Stephen J. Raphael, more myth than legend