The plane was then 4 hours late taking off.
On this basis you may be entitled to SOME compensation under the rules of the European Union. Go to the following link:
http://www.euclaim.co.uk/
As to the matter of charging you for overweight on the basis of your carry-on: Write Transport Canada. They maintain an airline consumer advocacy unit. I don't know if they can help you. The airline MAY have been within its rights. However, it's worth a try. I'm not sure what you meant by
Strangely, I was also advised against complaining about this.
If this means the airline was threatening you, include a photocopy of the airline's reply (with your letter to Transport Canada) which includes this comment.