For the rust removal on her tail I remove the metal and then scrub with toothbrush and toothpaste. Personally I take the metal waster and clip off to prevent future issues and store them, putting the tail back in with a white or clear cable tie. Others I know add the washer back on the pony. That's a personal decision.
For the rust removal inside the body, I fill the pony with denture cleaning tablets [one broken into pieces] and the very very warm water. This is usually enough to loosen rust - you ll need a pipe cleaner or a childrens toothbrush to get inside the body and clean the rest of the way.
The cancer spot is going to be a problem as while a pin prick and acetone might reduce it, it is pretty much unfixable.