1. It matters because oils oxidize rapidly (and can begin breaking down) in the presence of high heat. You want the oil to be relatively fresh while using it to cook. This is less of an issue with highly refined oils, but a problem with most natural oils.
2. Oil isn’t affected by leidenfrost, but the food you are cooking is. This effect prevents the food from falling below the oil layer and sticking/burning to the pan when the oil is moving due to water boiling off. When water molecules in the food come in contact with the surface of the pan below the oil, leidenfrost forces them to float back above with the food and allow oil to flow back into the space.
Moving food as you cook it helps with this as well. If you cook at the wrong temperature without this effect, oil gets displaced from below the food and it sticks. This is especially difficult with very wet foods with minimal structure like eggs.
No cooking oil is going to appreciably oxidize in the 70 seconds it takes to heat a pan. I cooked at very high levels for many years, I have seen a cook get fired on the spot for handling a pan incorrectly, no one cares about whatever oxidation is happening during preheating. The advice to add oil to the hot pan is so you don't forget and overheat the oil, or start a fire.
The advice to pre-heat the pan first is because if you forget an empty pan on the stove you get a hot pan, if you forget an oiled pan on the stove you get a grease fire.
Also try out carbon steal for a more nonstick type situation. Stainless is best when you want some sticking, like for building a fond for a pan sauce. Carbon naturally seasons similarly to cast iron, but much lower maintenance because you can wash it however you want and essentially rebuild the seasoning from scratch for each use.