The dude suffocated. You don't need anything near "instant" to prevent that.

Edit: Since apparently some people need reminding, per the article he had time to say goodbye to his wife before he lost consciousness, this wasn't some liveleak skull splat type thing.

The chain apparently caused him to be hurled across the room. We don't know how he died, but given the inverse square law, the possibilities are quite grisly.

He was wearing a twenty pound necklace. In a magnetic field that strong? His throat was crushed, likely instantly.

Do you have a source for that? The BBC just says "a medical episode" of which he died later.

Multiple heart attacks.

That I can well believe!