MPG. Can you beat this???

The plug fouler isn't a fix - It is a HACK. Exhaust leaks between the combustion chamber and the downstream CAT sensor is going to give the ECU erroneous data. That's part of the reason you can't get rid of the P0420 and it's more than likely part of your low mpg issue too. The upstream sensor is part of the mixture control equation.

 

Carbon at the tailpipe is a bad sign. Firstly, your engine is (or at least was) running poorly (producing too much soot) and secondly, the CC was not able to clean the exhaust.

After your most recent work, it will be interesting to see if the CC will be able to clean itself up assuming the engine is now running cleaner. Some long runs to ensure the CC gets nice and hot will certainly help in this regard. I'd sit on the P0420 for a bit and observe your engine PIDs to confirm things are better up front.

 

You need to diagnose all systems and engine parameters.
The main reason for incorrect engine operation is incorrect fuel supply settings. Your fuel flies into the pipe without burning completely in the engine.
Feedback, adjustments do not work.
If you do not set up the engine correctly, the high-voltage battery will not help.
It takes energy from the engine, not from the surrounding space.

 

If you are still showing a negative voltage on block 1, then yes, your HV battery is still a problem. Most likely the sensing wire has a break or the plug/socket have an issue. I'd like to think your 'rebuilder' was good enough to not put a module in backwards, but I'd also assume if one was backwards, it would be a bit more exciting than just a weird voltage.