Hi,
There are no vanes in a magneto (
AFAIK) but there are in vacuum pumps and I have been led to believe they can fracture if you turn the prop the wrong way around.
More clever people can confirm this or not. Never felt the urge to try it but equally noticed that when an engine had run backwards (happened when it kicked back) the vacuum pump was not immediately 'kaput'
The turning back of the overprimed engine is indeed a good remedy but usually only applies to handswung engines, which most likely do not have a vacuum pump as ancillary. Just a thought.
For most engines with a starter the procedure for clearing an overrich/overprimed situation is to start it with the the throttle open and the mixture at ICO.
FD