I Still do not see how the air can escape until the t stat opens. If the air cannot escape then you cannot completely fill the block via the radiator. If the T stat is not sitting in hot coolant, but hot air, it will take longer to open.
I've had my block drained twice. The first time I used a super stant, and barely got a gallon in the radiator and lower and upper radiator hoses, and after a while the upper hose finally got hot and the radiator drained faster than I could pour more coolant into it.
The second time with the toggle valved Motorad, I poured 3.5 gallons of coolant into the radiator, and after I turned on the heater for a while, it finally filled the flushed and empty heater core, and I got the remaining .5 gallons in the system.
Lots of newer vehicles have purge valves and will blow the head gasket if the cooling system is not bled before getting the engine to full temp.