You don't actually see that addressed very often in these advice columns, but in winter conditions that gallon of water you're carrying becomes a block of ice and can rupture the container, which is a real problem if it then melts and floods your trunk. I speak from experience. Use good containers and allow for expansion.
I keep my water containers in an appropriately sized ice chest, it helps keep them from freezing, and it contains the water if they do a freeze/burst/thaw cycle.
People in super cold climates like Minnesota, have to assume that in their case, the water will freeze no matter what, and they need to allow for the containers to swell without bursting, as you pointed out.
There are many collapsible containers which will hold water (or wine;) and are good for the trunk of the car.
Quite true. So carry a few cases of bottled water instead. I freeze the 16 oz bottles all the time and haven't yet had one rupture.