I think the key to catching scum!Galz was always reading between the lines; his posts will sweet-talk you or impress you into submission, so ignore the language and break down what he's saying into humanspeak.
Frustrating response if you are trying to tell me that I didn't do that because I feel like that is the kernel of my case - his supposed reasoning is plausible but no amount of reasoning can explain away scummy waggoneering, especially if the person has a reputation for being able to talk himself out of anything.
I was just providing an observation as someone who played with the original Galz back in the day. I think you are clearly aware of the dangers of scum!galzria.
What you need to be careful of is making this characteristic of Galzria the player the "kernel of [your] case."
For anyone to say "This Galzria guy is a slick talker, he must be scum" isn't a good case; that's the same as saying "This Galzria guy is left-handed, he must be scum."
If his reasoning is plausible, it's plausible. If it isn't, or you think it is scum-manufactured, you need to focus on showing that. Arguing that "well, it's plausible but since Galz is a slick talker, it feels slimy and untrue" doesn't work.
In a bad attempt at an analogy, imagine the slickest used car salesperson you can, the ones who'll lie to sell you a lemon at a terrible price. It's a stereotype that works in this case. No matter what he says, you will mistrust it. Galzria is that salesman.
Now, if that salesman were to say "this here car is a beauty! Look at that red paint!" and the car is actually red, well you can't say he lied there, even if he said it in a lying way.
The way to catch scum!Galz is to find the lies, not the way he says them.