One of the connections between the AC input in the
#sonoff switch and the output was a black wire. I simply desoldered it.
The other was a
#trace on the
#pcb. So I cut it with a knife. Deeply.
But it still was connected! There was an identical trace on the other side, difficult to reach. So I grabbed the
#Dremel and cut a giant hole on that poor pcb

That is rough. And ugly, and unsafe... but it works perfectly!

Tested the switch with the tester: not only it still works, but the two "output" terminals are disconnected from the mains and now they behaves exactly as if there was a switch between them!