I suggest you remove the ECoG extension for now and revisit it if needed.
I wrote this based on some example files I got from the developers about 1.5 years ago. I'm sure things have changed since. The ecog.extensions.yaml was an additional file, I thought I added when I merged, maybe I missed it.
Regarding the utilization of ECoG files in Brainstorm, the developers informed me that there are two ways of storing the locations of the ECoG electrodes now: one by storing the electrode locations on the .nwb, or you can have an additional .nwbaux (or something like that) file.
What was supported was based on the second approach.