This argument hinges on counting the time value of money for insurers but not their customers. The investment money made on float would instead have been made by the customers if they didn't buy insurance.
I think you're correct in a very specific technical sense (measuring in nominal dollars, that is), but not in the sense that matters in the real world.
Not really. Each individual customer requires much higher liquidity than the insurance company does - it's fairly likely that you'll have to spend your entire medical fund unexpectedly, but incredibly unlikely that every single insurance customer will need to claim at once. Also, investing is a lot more inefficient at small scales.
I think you're correct in a very specific technical sense (measuring in nominal dollars, that is), but not in the sense that matters in the real world.