1. Before departure if booked with a TA, the TA is responsible for all dealings with the airline and passenger, so Amex should have called about the flight, not AC SE.

2. Last carrier is always responsible for checked luggage issues. DL should have at least provided the usual delayed baggage package (some money and an amenity kit).

3. All carriers advise not to pack money, jewelry, computers or other valuables in checked bags and thus limit their liability in this regard. I would include corporate papers in this category. BTW did you lock you bag? Since you were not flying via the USA this precaution should have been taken.

4. Since you used Amex as your TA assume you used an Amex card that had trip delay and baggage theft coverage so your target once more should be Amex, not AC.

Quite frankly, aside from the issue of AE miles for taking a non-STAR flight, AC has done everything it should have done and has no reason for doing any more.