Great submission and write up
As said earlier ect is doing fine and am absolutely convinced is not the culprit.
With Tdc and timing, you basically want to verify if timing is jumped or in a wrong position; thought as much earlier.
Timing is fine, when head gasket was being replaced we took the opportunity to align timing belt markings (dark spots) to camshafts stamped nodes. Engine start and run better afterwards; so timing is not a problem here.
Below is the link of obd output for your perusal
Do @nightflyr
You are not convinced the spark plug will be the reason right?