The answer is yes, this trial may come down to your word against hers, plus whatever other evidence the police can obtain. Since you were making a complaint against her for child abuse there is definitely motive for her to lie to the police. You need to discuss these issues with your attorney who will argue to the DA on your behalf.
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There is no way you can know all the facts yet. Perhaps the woman told the police that she say you driving the car erratically and swerving in and out of the lane. Did the police make any pbservations about you such as alchohol on breath, blood shot watery eyes, unsteady on feet, etc. If the case is based on the woman alone, it might be a weak case but the police may still have had probable cause to arrest you. I sugges you retain a good criminal defense attorney. A good attorney will search for surveillance cameras on the route you drove and at the parking lot to see how you were driving.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 385-8015 or via email at Eric@RothsteinLawNY.com. The above answer is for informational purposes only and not meant as legal advice.
The DA has to prove that you were operating a motor vehicle under the influence of alcohol; or operating a vehicle with a BAC over .08 (I assume that NY law is consisant with MN and CA law). The DA doesn't have to prove the keys were in the ignition or that you were in the car when the police arrived. The fact that you witnessed a crime isn't a defense, but it may help you in attacking the credibility of the witness who alleged you were in fact driving. You get the DA to see 'the holes in the case" by getting a good attorney in your area. You may have defenses, but you aren't going to get near enough information to properly defend yourself in this forum. Good Luck.