park city water bags don't crack or loose water when they freeze. albertsons carries them. You can do the same with platypus bladders though the caps aren't that great.

Your water stored in your vehicle will freeze and will go flat unless you want to spend thousands to ensure it doesn't happen. Just keep some of the powdered single serving flavors for the kiddos and deal with it for yourself. A hot hands and a sack will thaw a bottled water.

Flat water is still water, i've drank 3 year old water without problems and the storage conditions for it were anything but ideal

Keeping them separate has several advantages, you can change out flat water and not loose the money on the electrolytes, if you have a puncture you don't have a sticky mess, you would have a hard time washing an eye/wound out with gatoraide, if your giving aid to a stranded person and they are diabetic it could do harm, adding a sugary solution to your radiator/battery will cause problems if you have a vehicle emergency.

Water bottles will sweat so keep that in mind with where you put them otherwise you wind up with moldy blankets.