I am going to fight it, but any input with respect to how best to proceed would be appreciated.
The officer seems to think you were careless with your lane changing and merging it seems. Once on the Gardiner Expressway West from the York Street ramp, you have to merge to the left to avoid being forced to exit on Spadina as the right lane is quickly marked as the exit. This is even more difficult when there are people to your left looking to exit on Spadina. You merged to your left, decided to pass on the right in the exiting lane, and then you would have had to merge to the left again. The lane lasts 700ft before it exits. I'm assuming the officer deemed your actions as careless to those on the left and right of you looking to merge in this short distance as you are using it as an opportunity to pass.
If you're looking to go to trial, you will want to request disclosure once you get your notice of trial back in the mail. You can start there. Also, I'd suggest you consider a plea deal with the prosecutor. Careless Driving isn't exactly small potatoes. It might be in your best interest to plead guilty to something a little less drastic.