Is this really how this works? I was under perhaps wrong understanding that once the TA booked flight is 24 hours or less to departure, it is under the airport control and then the carrier responsibility, particularly once online checking completed.

Having said that, I am a heavy Expedia user (as I can save £££ on various OW flight combinations). I likely did at least 20 flight segments in the last 12 months via Expedia. I had to use the Expedia chat bot a few times and they were very quick to pick up (less than a minute) and the folks I talked to were very switched on and resolved the issues on the spot. Not sure if that is because there is a different support for Gold (now OneKey Platinum) , but it has always been far easier and quicker to resolve issues with Expedia than BA support. Also I found Expedia refunds to be much quicker than BA (often same day or the following day)

OP: try the Expedia chat bot support, you should be able to talk to a human. If you go to the page on Expedia for the Flight Details, there should be a Contact Support button or such.