I think the MLL vouchers that come with the TD Aeroplan Privilege Visa is only good if you're flying on an Aeroplan reward ticket.

So long as you're the primary cardholder, and your Aeroplan no. is entered into the PNR, you are entitled to AC's priority services and complimentary checked bag when flying AC, no matter where and how you purchase/obtain your air ticket.

Since you have an Amex The Platinum, at YYZ, I shall go to the Amex lounge even if you have access to the MLL. The Amex lounge is 250% better than the MLL.

Unfortunately, there is no Amex or Priority Pass lounge at YUL domestic. You may want to check what is available under TD's Global Lounge Service at YUL domestic.

No sure about fast track security for TD Privilege but you can certainly use the Amex queue.