No oil on the plugs... the valve cover gasket is fine.
I was expecting to have to replace it before.
But I lucked out on that.

CEL... yes that came on today... twice (more later)
Rough idle... yes... real rough now.
Backfiring... no.

I had not thought of the converter, could be possible. I have been getting a vibratory noise from the exhaust during cold morning startups... just figured it was age and cold though.

I'll add the converter to the list of possibilities and have a look at it in the dark while hot.