So Durkon knows it's Tarquin now...

[Agnostik]

Elan would know that circumstantial evidence can be a certainty....

Actually, it's the opposite. The more circumstantial evidence you have that something is true, the bigger the chance that it's not actually true. In fiction, obviously.

Just like in the Elan's observation you're referring to, the less likely something is to be true, the bigger the chance that it IS true just to service the drama.