It certainly sounds as if you have a defensible case. It can't hurt to get a free consultation from a locally experienced defense attorney. Obviously the best outcome is to beat the case. If convicted, the punishment may depend on your probationary status (if you're on probation) and if so, whether the terms prohibit the use or possession of alcohol. Best of luck.
NO, you never have to plead guilty. The outcome of the case clearly depends on your prior criminal history. Under Chapter 18B, the open container violation is a Class 1 misdemeanor which is punishable by a fine or up to two years in prison.
You should consult a local attorney very soon to protect yourself and ensure you don't get a faulty conviction!
The information provided herein is general information and not designed to create a lawyer-client relationship, nor is it intended to be all-inclusive. You should seek counsel from an attorney about your particular situation to ensure your rights are protected.