Old oil on cast iron Dutch oven

With enameled cast-iron, I don't bother to heat the pot before adding the oil. For something that's open-pored, like plain cast-iron or carbon steel (like an old-school crepe pan), it's better to preheat the pan. I can't see any chemical advantage to preheating enameled, as it's inert and non-porous. Of course everything DOES need to be properly hot before you add the meat.

I can take or leave letting my roast get to room temp when doing a braise. It's going to be in the oven/pot long enough that it's going to be cooked to 200+ degrees all the way through no matter what. The only possible advantage of letting it warm up on the counter is that it will take marginally less time to cook. And by marginally, I mean you probably won't even notice, on the scale of pot roast cooking time. For a steak or a rib roast where you want the middle warm without the outside being totally dry and charred, room temp helps a lot.

The only real tip I can give on pot roast is to use the right cut of meat. You want something from the shoulder, which usually means chuck. It has an excellent mix of rich meaty flavor, collagen/connective tissue (which makes that gelatin thats critical to the silky mouthfeel of pot roast and BBQ), and fat. You can also use something from the leg (shank) or tail, which also have loads of connective tissue, but they're usually a little low on actual meat. If you get the right cut of meat and cook it low and long enough to get that collagen rendered, it's pretty hard to mess up the rest of it.

You're going to die horribly from cooking in a rusted pan!

Just kidding! A little iron in your food isn't going to hurt you, and can actually help prevent anemia. To quote On Food and Cooking (pg 790): "Excess iron is readily eliminated from the body, and most people can actually benefit from additional dietary iron."

Now, to back this up further:

You won't get much iron out of the pan unless you cook something acidic in it. Rust is insoluble in water without acid present, and in order to become soluble you have to convert to iron nitrate, sulfate, or chloride, according to the solubility table. Nothing you cook in there is likely to cause the necessary reactions to render it highly soluble.

I found a source quoting specific numbers for iron from cast iron cookware, if you are concerned. The gist is that most cooking in cast iron added from 1 to 5 mg of iron to the food, with the highest numbers coming from acidic foods with tomatoes. Applesauce was the highest, good for about 7 mg.

This level of iron intake is quite safe and healthy. To give you a point of comparison, the FDA suggests iron intake of 8 mg/day for men, and 18 mg/day for women, and 80% of the world may be iron deficient. Iron toxicity occurs at about 45 mg/day. So, as I initially stated, you're perfectly safe cooking in your cast iron, and are probably helping your health rather than hurting it!