Here’s my 2cents. In the beginning of video you have the ‘big blue spark’ on the last switch on right furthest from stack screws.

1st, tighten switch stack screws if you haven’t already. If loose or slightly, you will have to check switch gaps again. Not much room for error on the gap on those switches.

Make sure all blades are in appropriate slot. (which they appear to be)

I believe you wrote you cleaned contacts. On the switch I suspect with spark, use a metal file followed with a flex stone file. You may have a pit in one of the contacts on that switch. Inspect closely with magnifier if you can. Hope that may get you going. Good luck.