I been trying to find the best possible solution to doing an audit report to find Overlap, Invalid and Duplicate spans, but have come up short on the Overlap.

What I have so far is building a formula to compare the first row and the second row of the effective date and term date and mark if the second effective span is within the range of the term date but not having much luck.

you could try to use next fiction.
something like
if date_field >= next(date_field) then "overlap" else "na"