Let me begin by urging you to retain the services of a criminal defense attorney in your area. An experienced attorney would go a long way towards protecting your rights, especially when you may have a lawsuit at the end of the criminal prosecution against the law enforcement officer.
You do have some possible issues for defenses. An assault is typically defined as an intentional act to place someone in reasonable fear for their safety. Based on the facts you've provided, it seems it would be difficult to prove you had an "intent' to do anything. Additionally, it would also seem difficult to prove that YOU could have placed anyone in "reasonable" fear for their safety. You alluded to another defense issue, that the officer only brought forward the charge AFTER you threatened to sue. All told, it sounds as if you have some excellent issues for a seasoned criminal defense attorney to explore.
As to penalties if you are convicted, it is certainly difficult to predict an outcome. However, with what seem to be weak facts, and assuming you have little to no criminal history, I would suggest probation only, no incarceration, would be a realistic result. With NO criminal history, perhaps just some fines and court costs.
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