Since you are trying or have tried numerous solutions, you could check and see if the cassette is tight to the hub. Use a Park FR 5.2 and make sure it is tight. I know you say there are no tight links but I thought the same thing as the poster above when I saw your excellent video. Do you have an older chain just to check it out?
I also disagree that the cassette or 11 tooth cog in particular is worn. You would have to be riding in that cog everyday for hours since you purchase it to be worn. It may wear twice as fast but I don't believe that most people use an 11 tooth cog twice as much as the others. Good luck.