An update - I requested for the DOT to intervene after NOT hearing back from AC for nearly 3 months.

"Please know that Customer Relations has received your correspondence from the DOT regarding your flight from Dallas to Toronto on June 18, 2022.

We are very sorry we were unable to accommodate you in the Business class cabin due to a broken seat. Please accept our sincere apologies. After further investigation, you were denied boarding due to a weight and balance requirement for the flight. In accordance with the Air Passenger Protection Regulations, compensation does not apply under these circumstances as this would be within a carrier’s control, but required for safety."

Compensation (travel credit) was offered for the one day bump. I will probably move on from AC after this incident - the kind of response does leave a sour taste in my mouth.