Your only real avenue of escape is to ask Hertz to look up the previous renter's record and find out the return time. If the return time is after the traffic ticket, you got a case. If the return record shows something different, sorry but it's over. I bet dollars to donuts the previous renter already returned the car and someone you don't know got the ticket. Maybe it's an employee driving to the car wash and got a ticket and they decided to transfer the traffic fine to you. I've never been to Calgary, but Vancouver doesn't have gates with cameras at their rental car lot to prove the start rental date and time. Maybe you can get photos of the driver from the ticket and send the GM of the facility those pictures to see if it's an employee, to see if the GM is honest, and to see if the GM likes the employee more than a foreign customer. The corporate HQ is going to think it's you unless the previous renter's return time is after the ticket. The credit is just a customer service gesture. It doesn't necessarily change the start time of the rental.

This is one of those cases where you were in the right, and you'll probably still lose even in court. That rental start time really hurts you. Be prepared for the outcome to be pay up and move on. Sorry.