I believe water mains are breaking due to thermal stresses unrelated to complete freezing. It’s not thermal shock per se. But there are miles upon miles of metal pipe that’s contracting in ways it hasn’t experienced before or since it has handled many thermal cycles in the opposite direction. It’s a subtle, but effective means of damaging things.
On the household side, shutting off the incoming water and opening the taps is only effective if you have bleeders at the low points to drain off the water within the pipes. Otherwise you still wind up with enough confined water, that when it does freeze, it splits the pipes quite effectively.
As for Texans having adequate firepower on hand to be able to defend themselves from marauding bands of thirsty, dehydration-crazed poor people... as I understand it, most people of means have fled to the beaches of Mexico, leaving their supplies and poodles unprotected. I’d be more concerned in the potential for a surge in firearm suicides during all of this with roughly 70% more people who take their lives with a gun every year in the us than are murdered by one. Goodbye, cold cruel world.
Personally, if society collapsed to the point that I’d be in a position to have to shoot someone who needed a drink of water, I’d probably eat a round myself. But that’s just me.