I think the problem I have with your approach, Soter, is that your character often tends to simply ignore the CCP-defined nature of the Federation. It isn't, according to lore, a perfect classically-liberal democracy. Actually, it's often (aside from TonyG materials) portrayed as a near-dystopian Brave New World-style democracy with the masses anesthetized by entertainment and excess. And the Caldari State is 1984, the Amarr Empire is the Church Militant With Lasers, and the Republic is Sudan.
Ah, wait, I've been reading too much Warhammer 40k again. Perhaps things aren't that bad.
But humor aside, the fact is that CCP has repeatedly stated that the Eve cluster is a dark and uncaring universe. You can't have a warm and idealistic Federation/State/Empire/Republic in a dark and uncaring environment, unless of course they exist solely to be crushed like bugs. Since none of the four empires are getting crushed like bugs, and since CCP does not characterize any of them as shining examples of humanity, it's logical to conclude that they are all deeply flawed.
What does one do when one is trying to defend a deeply flawed organization? Especially when, thanks to FTL communication, one can have examples of the flaws immediately? Well, the same way one justifies flaws today: excuse it, exalt a flaw as a virtue, ad hominem the accuser, etc.
This is one reason I love Seriphyn's SDII rp - he takes one of the Federation's flaws (extremism/hypocrisy) and makes it a virtue (to himself). This happens in real life - this is realistic - and it's very interesting to watch a man who believes himself fundamentally decent and humane do horrible things in the name of something he believes is good. Could his character but realize it, he would see that he is being exactly like Tibus Heth or Sansha Kuvakei, but he is so blinded by his self-righteousness that he doesn't.
That's great RP.
But your character's consistent response to his rp and his actions has been denial and to undercut him at every turn, and I can easily see how Seriphyn could assume that this stems from an OOC desire to eliminate any negative portrayal of the Federation or its policies. I can see that, because that's exactly the way someone would treat Seriphyn if they were trying to undermine his RP.
Now, I'm not trying to attack you personally, here, so let's be clear that I'm assuming this is not intentional.
I can understand replying to someone in this way if they create unreasonable situations: if someone were to post stating that Tibus Heth had personally expelled their family from the State after they caught him embezzling cash, I'd be the first to call them nuts. But Seriphyn's RP is not unreasonable. In the past, we've had the Federation attempt to take over a Caldari station, "disappear" teachers for not toeing the party line, burn an admiral alive in front of thousands, and a long list of other actions that make the antics of Seriphyn's SDII look like well-reasoned tactics.
I don't automatically reject as a liar anyone who posts something bad about the Caldari State. Most of the time I excuse it, or justify it, or argue the point, or any number of other things. This allows our characters and theirs to both defend their points of view without anyone getting the "your RP is wrong" message, which is good, and letting us put forth our disparate visions of what a faction is.
I think there are many opportunities for you to do this in the current situation. You could petition a Senator to investigate these activities, or work with Seriphyn to implement an arc where Soter tries to cut down on the SDII's excesses. There are a million and one ways to resolve a problem other than "you are lying", which is, in any case, hardly fun as far as storytelling goes. There's no need for antagonism here, especially amongst players in the same faction.