This post argued that cast iron is the Michael Jordan of cookware. Cast iron is neither particularly effective, safe, or convenient. In this way, cast iron is the Kurt Thomas of cookware: mostly just cheap and predictable.
1&2. Generally, thinner cookware will transfer heat throughout the pan faster. Cast iron is about the opposite.
4. Yes, and the quantities involved with cast iron are frightening. Teflon requires a single administration of about 0.005g or less of material. If you've ever scraped a cast iron pan down to the metal, you will get a gram or two. And that is coming on and off all the time, plus given how permeable it is as compared to engineered surfaces, it's going to exchange a lot of material with the food itself.
With regards to the health risks of seasoning, first, seasoning is produced by polymerization. I misrecalled; it is linseed oil (highly unsaturated and unstable) which produces the best seasoning:
And of course it is linseed oil and other highly unsaturated oils that produce the nastiest polymers at high temps similar to pan seasoning temps:
I personally use thin blue steel (Acier) for searing and Green Pan (very thermo stable) for non-stick.
Cast iron is absolutely the crappiest approach to cookware. 1. Cast iron does not transmit heat well, leaving distinct hot spots. Sprinkle flour on one, heat. You'll see it doesn't brown evenly. 2. Cast iron sucks for searing-only applications. A thin blue steel pan heats quickly and much more evenly. 3. Cast iron is a pain in the ass to maintain. 4. Cast iron is potentially dangerous. Try and even guess what that "seasoning" is made of. You wonder how you get the non-stick surface? Intermolecular diels-alder on the unsaturated bits (which is why flaxseed oil is the best seasoner), producing nasty cyclics and thousands of different products. In quantity, the stuff would be considered toxic waste. Instead, you put it in contact with your food on a daily basis.