Bags can travel without their passenger if it was not the choice of the passenger to send their bag without them. Thus, the airline can choose to transport any bag as they wish, as long as it is the airline’s choice. There is no risk and the bag goes through the normal scanning.

Good to know, I never realized they used passenger planes for cargo as well.

Lufthansa did.

No, it was not prenegotiated, and I certainly wasn't expecting, until it happened, that I would end up with new clothes that would be of no subsequent use to me. That was perhaps naive, as I find it hard to find clothes in my size and style even at the best of times, and rushing out as soon as the stores opened in Berlin to buy a lot of clothes in a hurry was not the best of times.

I think the position of getting reimbursed for "necessities" is stronger if you purchase them then ask. If you sit there waiting for pre approval, they can argue your need is questionable. They won't assume you are naked and unshaven without the expenses. They will assume you're getting by. If not having the necessities would mean being naked and unshaven, buy then ask. (But worth checking what the limit is of the reimbursement you are entitled to under whatever law or insurance policy applies.)

Oh no, they would not let us be unshaven! When everyone from the flight lined up to make our missing-baggage reports, we were each given an official Lufthansa gender-specific toiletries bag. The male version contained the cheapest, nastiest, bluntest shaver I've ever met in my life. So I was not unshaven, just shaved very badly and bleeding slightly.

As to OP's situation, as the last delivering carrier, it is AC's responsibility to deliver the bag and to compensate OP for losses. This is the case whether it is UA or AC which actually lost the bag.

By this time, there should be a WorldTracer number for the bag and that is where I would spend my time: https://wtrweb.worldtracer.aero/WTRI...A-C6127780624D

Calling AC, visiting airports and the like is a waste of time. The AC people with whom you speak are looking at the same screen you can see for yourself.

At the extreme, the bag is considered lost at 21 days. At that point, AC is on the hook for the fair market value of the bag and its contents up to a maximum of approximately CA$2,050. Anything AC gas reimbursed for incidentals up to that point comes off that amount.

For this reason, insurance which is simple is the best way to go. Many will pay out specified amounts without particular hassles and given that time is money, you may find that the easiest way to go. The insurer then deals with AC (or not as it chooses).

As a final note, OP states that he did not have a change of clothes. Given liberal carry-on allowances, it seems prudent to have at least that with you in a day and age when many travel with no checked luggage for 7-10 days without even thinking about it.

Was it white and black? Likely the Intl J ones with one of the washroom shavers thrown inside ;)