How much did you try and push this with the security agents?

Technically you have met all the rules for entry into the domestic concourse. That said there is a training manual section that has Security agents directing passengers to the most convenient security area. When I have requested to override the customer service suggestion, I usually have to talk with the Supervisor. Note this CS override does not work for instances where one cannot get to their gate from security lane (Example use domestic security with a BP for US preclear area). However with Toronto T1 you can enter Internation concourse from Domestic side and remain entirely behind security.