How to prevent my disc brakes from locking/sticking due to debris?
I think that cleaning with water is the problem. You wash them overnight, leave the bike outside. Water freezes inside of the breaks. You splash hot water again in the morning and get more water on your breaks and behind the pads that will freeze later on. 

I would try avoid using water in your daily clean. Just try it for couple times. Clean the pads with dry cloth and make sure there is no water in the system. And see how it works next day.

To be honest, I have never experienced this kind of problem. And I've used disk brakes for the last 10 years in very different conditions, including thin wet mud all over the bike, brakes and pads. And I never got mud between pistons and break pads.
So I think the problem with "magnetically held" pads. My pads always been held in place by a spring.

So if you have this option, try changing the breaks, but make sure that new breaks hold the pads with a spring, not magnets.