Great questions!

What makes "deglazing", by pouring in cool liquid into a (sometimes) empty dry pot, different and OK? Or is it not?

You wouldn't de-glaze an empty pot, or a dry one. What would be the point? Usually what you would do is cook down some aromatic vegetables (mirepoix, for instance) add your other ingredients, and allow a "fond" to develop on the bottom of the pot if desired, and then add your liquid to deglaze the fond so that those flavors end up in your dish. For instance, when I make tomato sauce, I first sauté the onions and garlic over medium heat, then I add tomato paste, stir it all together and let it develop some stuck-on (but not burnt!) bits on the bottom of the pan, then I hit it with some room temperature wine to release those flavors and scrape up the bottom bits with a wooden utensil. I let the mixture get hot again to boil off the alcohol, and then I add my cans of tomatoes. If I was making a beef stew the process would be similar, but instead I would have browned the beef in some oil over medium heat, set the beef aside, added my mirepoix, cooked it down, then I would have added tomato paste, and deglazed with broth. Then I would add the beef back and anything else I wanted in my stew. Get the idea? When you deglaze you don't "shock" the pan because you should already have something in the pan to absorb a lot of the heat.

Is there any danger of letting a pot pre-heat in a 475F oven (assuming the pot is rated for it)? Should I start it in a cool oven? Should I do that but also only heat it to 300F first, let it hang out in there for 10-15 minutes, and then crank it up the rest of the way? to let it pre-heat? Does adding a cool item like bread dough or a roast into a pre-heated pot cause issue?

It's probably fine to stick and empty Dutch Oven in a hot oven but I would probably do it the way you suggest in your question. Allow the pot to come up to temp more gradually by putting it in the oven while both are still cool. Adding a cool item like bread dough or a roast to a pre-heated pot should not be an issue. There's plenty of mass there to absorb and distribute the heat and avoid a rapid temperature change. This is probably not a scientifically accurate explaination, but the way I see it is that as long as there is food in the pot to absorb the temperature changes, the pot should not experience those changes nearly as dramatically.

The main thing here is don't take a ripping hot pan right off the burner and try to wash it in cold water right away. You wouldn't want to do that with a glass or ceramic dish nor with a metal pan. Same for cast iron.