Since the air leak appears after the pump is shut off, the air leak could be anywhere including the filter.

One thing to note is that this began just after a complete tear down and cleaning of the filter grids (DE filter). If the o-ring on the filter was not seating I would assume i would see water running down the filter as air was being introduced.
Not necessarily. Sometimes leaks can be one way only depending on the geometry. This is especially true with o-rings because the high pressure when the pump is on will tend to seal the o-ring.