And you forgot that nobody in Luria had access to the message that was ultimately forged.
It is possible that there was a second-hand process. In that case, it could also be a malcontent in Corsanctum. Or maybe a Fissoan.
I'm talking about reality, though. And in reality, the Lurians probably wish they did this, but they probably actually didn't.
Well, consider that the letter forged was sent to a group Garret was a part of. While it's possible that the writer was part of this group, that would mean he's either stupid (or recklessly ignorant, as Garret did send many letters in the past) or should have at least expected for the truth to eventually come out (because the real letter would have been spammed by 40 different people to the Zuma). However, if the person wasn't part of the 'moot, then he might not have realized that Garret was part of it. Which would make this very important error make more sense. As if the person who sent the forgery to the Zuma is actually part of the 'moot, then the Zuma can know for sure that he wrote the forgery himself. And that's a pretty big risk.
We don't know if the person who sent the letter forged it himself. However, it is obvious that both the forger and the sender, if they are not the same person, want ill to the 'moot.
Is it the Lurias? I don't know, could be a bunch of different people. But honestly, I'm kinda hoping that they get blamed regardless of who actually did it. I tire of having that big threat that is always just waiting for an excuse to pounce on us for no good reason.