I have never heard of cast iron being poisonus but I have known that the cheaply made pans can certainly do it in my opinion.
I have never heard of sanding cast iron with sandpaper just to get the seasoning off. I've always seasoned my CI once a year and if the foods taste metallic just reseason the pan.
Location: I live in the Heartland of the United States - Western Kentucky
The only way I've known to remove seasoning completely is to "fire" the item in a campfire or such until it's almost red hot, then let cool and start from scratch. Never heard of the sanding technique.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!