Macbook Air 2013 causing Wifi Drop
Why not have the Air repaired or replaced if it’s causing you interference?

I don’t think we’ll be able to review all the extensive work you’ve done without being in the room with you so unless you think you have marked something as DID NOT WORK when you really didn’t complete those steps, I think this is pretty clearly that you need a router that can accommodate interference from a device that’s failing or you need to remove the failed device from the environment that can’t tolerate it.

Also, a proper wireless survey might find out the details, but the costs and experience are high to do a proper analysis of the actual radio transmissions with WireShark or actually measuring the RFI.