Looks like the root cause of the runaway was that MX was troubleshooting the messages caused by the chaffing, decided to defer the CA's split trim switch, and re-installed it upside down. They didn't op test it since it was deferred.
Technically the CA should have been able to use the good trim switch, but both switches were in the same assembly, so both were upside down.
The wire chaffing caused the EICAS message which led to the MX troubleshooting and error.
https://www.youtube.com/watch?v=iMMQ6-OfnL0