The original actress cast as the Wicked Witch had a severe reaction to the green paint and had to be re-cast.

This is incorrect. She wasn't re-cast, but Margaret Hamilton did have to have a liquid diet to ensure she did NOT ingest the green copper based makeup, because it was indeed toxic. No actress would have a good time ingesting it.

She was then caught on fire while exiting down one of those trap door drops and had to recover from that, but recover she did and completed the movie. The harrowing tale is that when she caught on fire, and they put the fire out, they had to immediately scrub her skin where she was burned with alcohol to remove the green makeup lest it seep into her skin and disfigure her. She had second degree burns from the incident, and refused to do any other scene for the rest of her career if it involved fire.

You're probably conflating with Buddy Ebsen's exit because he DID get very sick from inhaling the silvery aluminium dust makeup they first used, and almost perished. He had to be replaced, he would not be able to make the film. They changed the makeup to be a paste, and recast the Tin Man with Jack Haley.

But if you listen closely, you can still hear Buddy when there's a group singing, specifically "We're Off To See The Wizard" group version. MGM chose to save money and not re-record some things with Jack's voice. So Buddy's still in the movie, just not seen.

/Movies had to learn unfortunate lessons. Lot of unknowns/ignorance/impatience/savings of costs happening back then... and even still today. :|