> he is implying that the focus on the republican cases (the survivors) is a red herring because the system is presumably such that an actual conspiracy cannot be detected.

Maybe GP is implying that, but that's not what they said, because what you describe is not survivorship bias.

> I don't know anything about elections, a question worth asking is what would it look like if a conspiracy set out to exploit mail ballots? could detection only originate from a defector?

You are asking the wrong question. The correct question is: If mail-in voter fraud cannot be detected, then why do people claim that mail-in voter fraud exists? If there isn't evidence that it exists, then the people who claim that it exists are making statements not based on evidence. In other words, their opinions are either not informed by facts, in which case we should disregard their opinions; or their statements are not sincere expressions of belief, in which case they are liars and we should not trust them.