Update to AFCI problem....

So, I identified the receptacle that had the main run from room to main panel box, it had the one input from the main panel and two outgoing lines to each side of the room...(All three wire 12 gauge yellow jacket)

First step: Disconnect all but the main run to the one receptacle.....

1) Fantastic, it works, no tripping even under heavy ac motor load.

2) Hook up first branch....it works! even under heavy load....so the main run and one branch are A-OK.

3) Hook up second branch, put load, POP breaker opens.

4) Disconnect every receptacle in series then hook them up until failure...First receptacle...fine...second, POP under load!

5) The second receptacle branches twice more, disconnect

branches so only link between first and second receptacle...POP under load

Now the sad news, the connection between first and second receptacle on the problem branch is 12 gauge romex that travels at least 15 feet, up and over a door...

NOW WHAT!?!!?! I have identified the problem link, but man, how do i pull a new cable through? Tear out dry wall? Do I have any options here?