Not too sure where the OP is up to in their fault finding progression, but, to me, a few things don't add up, or maybe I missed something?
Firstly the fault seemed to come on about the same time and the breaker couldn't be reset for a few hours.
Then while testing the breaker could be reset, not having to wait. What changed?
Placing the unit in Direct mode stopped the unit from tripping, OK could be a leakage fault, but initially, if it is/was a leakage fault, it seemed to clear after a few hours and the breaker could be reset. Strange fault (Ahhhh, those red ants) as the leakage fault seems to clear.
So at the moment, the SafetyCut doesn't trip in the Direct switch connection.
Now the OP is looking for a leakage fault by isolating loads and turning off breakers. Is that correct.
I'd still replace that SafetyCut with a newer unit.