Idiom or phrase or proverb for When ever you need me you conveniently declare me the head of the department (of course an acting head usually to throw me under the bus to deal/make difficult decisions or give the bad news), other times i am no more than a no-body (or usually treat me like a slave).
Hello I believe "Scapegoat" is what you are looking for. Definition from Urban Dictionary:
(n.) A scapegoat is an event person or object that is used to lay the blame on for all that goes wrong, regardless of the contributions of others.
Example: "You are just using me as a scapegoat when something goes wrong"
Another phrase that could work is "Take the heat"
to take blame of or get punished for something, whether guilty or not.
Example: "You always want me to take the heat when things go bad"
Not an idiom, but this poem that seems to express what you are saying
It's not my job to run the train, The whistle I don't blow. It's not my job to say how far the train's supposed to go. I'm not allowed to pull the brake, or even ring the bell. But let the damn thing leave the track and see who catches hell!