The only assumption I'm making is that a prosecutor is charging a defendant with a crime. Is there something else that a prosecutor could be doing? You are arguing that because he might not have committed a crime he should have protections of his office (even if that protection only extends to where the question of guilt is determined). What I am pointing out is that if he is guilty then it could not possibly have been within the scope of his duties of office. Therefore moving the question is moot.
A = he performed an action
B = an action is a crime
C = an action is within scope of official duties
D = federal court is correct forum
E = state court is correct forum
You are stating that: If C then D to determine if A and B, but of course if A and B then NOT C. So by stipulating C, the question of if A and B is moot. Therefore the question of "if A and B" has primacy and therefore E to determine if (A and B ), NOT A, or (A and C).