I drained all coolant from radiator (using red plastic drain), and engine block (using hose going into the oil cooler behind oil pan)
I assume the only coolant left in the system was heater core.
Upon refilling, it only took 1 1/2 gallons of coolant mix. Is this right? I thought I read the cooling system was 3 gallons. No way 1 1/2 gallons was left in heater core. The car reached normal temp no problem, and ran fine.
Hmm sounds a little strange to me, are you sure there's not a big air-bubble in it?
I'm not sure how to remedy that the right way, I'd try squeezing the hoses to pump the water around a bit.
What car is it ... assume it's a C30 judging by the label, is that correct?
When I Google "Volvo C30 coolant capacity" the first entry that comes up is a Pelikan Parts page, and they say "you'll need roughly one and a half gallons of coolant" ... if I'm honest I don't even know how much a gallon is, but it sounds like your 3 gallons might not be correct? (I also Googled the conversion from gallons to litres ... 3 gallons would be about 11 litres, that does sound a very large amount to me but maybe whiteblocks are unusual - would be better if someone with experience with them commented here 
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