Great write-ups, I love all the pics.
I do have a comment though. The danger of flushing with water like this is that you can never drain all of the water out of your engine. You guys need to be careful, as you are adding premix (50/50) coolant to your system, and there is a bunch of flush water still in the block and heater core which will dilute the 50/50 mix, which is not acceptable! (In Canada that will evenutally cause a cracked block due to the really cold temps in winter)
To darylzero's point, if you think you should have been able to get another 2qt into the system, that probably means there is close to 2qt of your 'flush' water in the engine. In reality, it may be closer to 1qt, and the other qt is air bubbles. But do the math, if 1/7qts is straight water, that means overall your solution is no longer 50/50, it would be closer to 42/58. (3/6 (qts put in) = 50%, 1/7 water, = 3/7 50/50 premix, which is 42%)
I could be wrong here, but use a hydrometer to test the boiling and freezing point of the new coolant, and if it doesn't meet the spec, you will need to add some undiluted coolant to bring the solution to the proper ratio.