I can't answer the "If God sinned ..." because I don't think He CAN sin -- because I don't think Sin is something about which we can say "can."
For:
-- Perfect freedom seems to me to mean freedom without defect.
-- To choose evil is to manifest a defect in, um, the entire willing apparatus - that is intellect and volition and whatever else.
Feeble attempt at analogy: It is not a limitation on cleanliness to say a clean thing can't be dirty :: It is not a limitation on freedom to say the perfectly free God cannot sin.
We have to move carefully and slowly, but I think you may enjoy the possibilities here.
MUCH AGREE.