Uneven tension on Gates Carbon Drive belt
Addressing question 1.

I understand that to measure the tension you pluck the belt and app measures the frequency. Anyone who plays guitar will know a string sounds differently when is plucked near the bridge compared to plucked closer to its center. Plucking near the bridge suppresses lower harmonics.

The app instructions should tell you where to pluck the belt, but halfway between the chainring and sprocket is probably the correct spot.