Carbon buildup along the firewall?

So a month ago I had to limp back to home approximately 200kms, because my car had massive power loss and was suddenly super loud. I thought it was a head gasket, but it wasn't.

I replaced the catalytic converter, and I thought that my problem could've been the not so tight exhaust manifold bolts when I uninstalled the old one.
But it wasn't.

It's still loud, still doesn't idle right, hesitates when you give it gas and is still just as loud as it was before.

But now, there is a lot of carbon buildup along the firewall and anything under the fuel injectors.

See images

I don't quite no what to make of it but I need this car for once to just run normally.
Also, I ran the OBD and got the same 2 codes back that I've had forever.

P0122: H02 HEATER CONTROL CIRCUIT BANK 1 SENSOR 2
P0036: THROTTLE/ PEDAL POSITION SENSOR SWITCH A CIRCUIT LOW

I've replaced the throttle position sensor 2x and the downstream HO2 sensor.
Never changes or fixes anything.

Could it be the crankshaft position sensor or the camshaft position sensor?
And I found the other end of a plug that had no connecting wire. What goes there?