Since every game he plays in, he adheres to "Be as honest as possible" and "Use a lot of analysis," the best way to catch him is to work backwards. I've had success with it before.

Basically, instead of looking at all of his actions, and going "Were these the actions of scum?" you instead go "OK, assuming Isaac is scum, what was he trying to accomplish with his actions?"

If you just look at Isaac's posts, you'll say "Oh, these don't seem too scummy."

But if you go "OK, Isaac is scum. What was he doing?" You can see "Oh, here he was trying to seem useful, without giving away any information. Here he tried to spark a witch hunt, before he or any of his potential allies could be targeted"

Isaac will call this ludicrous, saying that it's way too vulnerable to confirmation bias- which it is. However, it's a good way of turning up possible suspects when you have few leads. If you do it for EVERY player, it sort of evens out the bias.

(If you'd like, do it for me. Assume I'm scum- what the hell would I have accomplished with a long, rambling post talking about possible scum setups?)