How and why sleeping bags work is a much discussed topic, as you can see. However, the way any kind of insulation works is to isolate whatever you are trying to keep warm or cool from the outside environment. Whether it's your house, a thermos or a sleeping bag, it's the same principle.

Without repeating the obvious, a bag only works because it isolates you from the cold outside air (that is, air outside the bag). It doesn't matter how you do it. Some methods are more efficient than others at creating the dead air space you need, but that's it. A mummy bag works better than a rectangular bag because it is more efficient in creating a shape that will be smaller in volume and less likely to leak air in or out. A longer bag may take a bit more time to stabilize the temperature, but I doubt it is that big a difference.
Your head is a big radiator (one reason divers in cold water wear hoods) so wearing a fleece hat, balaclava or something similar will make a big difference.
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