I was behind two cars in a left turn lane. The advance green light went on and the cars ahead of me started to turn. At the same time, I heard a siren approaching (I was next to a hospital at the time). As I entered the intersection (travelling north, turning west), I realized that the siren was coming from an ambulance travelling westbound approaching the intersection. I also noticed an officer on the north shoulder west of the intersection, frantically waving the cars that had turned ahead of me to the shoulder. I assumed he was clearing room for the ambulance and quickly followed the other cars to the shoulder. Of course, he promptly wrote me a ticket for improper left turn, even though I thought he was directing traffic. I tried to explain this to him, and he told me to shut up or he'd give me another ticket.
Do I have a snowball's chance in hell of convincing the crown or judge of this?
It seems like you have a good story but you are missing the other side's. It's a bit late but you should be requesting disclosure before saying anything at trial. Most people are convicted because their defence strategy is either "I hope the cop doesn't show up" or "I'll tell my side of the story." They are going to be found guilty. Don't make this mistake. You need to know his version before you speak.
Also make sure you mention that the previous two cars were pulled over BEFORE you made your turn, establishing that the officer was motioning cars to pull over before you completed your turn. You may also want to add that you remember your written driving test asked a question about if the light is red and an officer is motioning for you to go through, the correct answer is to go through, not point out the red light to the officer. This is a similar situation. You know the proper lane to turn into, but the motioning officer should be obeyed first.
In cross-examination you may want to ask the officer about the other two cars pulled over establishing that he pulled over several vehicles at the same time which should strengthen your version of events.
Good luck and good fight!