I hated to bring this up initially because your original post said that there was good water pump flow capacity. I don't know how you measured that, so here is my $.02 worth.
Many years ago, I had a car that was heating up and after checking everything, I found no clue as what the problem was. Finally, I made a desperate guess and installed a new water pump. Sure enough, the impeller was slipping on the pump shaft and was only moving a fraction of the coolant it was supposed to be pumping.
Hope you are able to find the source of your problem soon.