One of the very revealing numbers will be the current in the EGC. That is most easily and safely determined with a clamp-on ammeter. ...
Agree problem circuit(s) better located measuring current.I like the amp meter approach.
@andyman
Open all branch breakers. Turn on one circuit at a time (disconnect all but one hot if MWBC). Verify current of hot to be greater than zero (higher is better) then measure current with both hot and its neutral (verify) through clamp with panel connections electrically on same side of clamp. If measure is greater than 0A, you have a ground fault on that circuit... or a faulty meter, which can be verified by clamping around an extension cord while powering a load. Measuring circuit current this way is the same principle used for GFCI protection.