Yes, the whole "I'm working for the humans. Wait, I lied, I work for Mundus. No wait, I lied again, I work for myself" theories and whatnot seem like a very far stretch.
The way I speculate, all members of the Order are ordinary humans who have become aware of the demons' in the city and have a genuine desire to fight them back and defeat them under the lead of Vergil. It's Vergil himself who might be the fishy one on all this. I share the previously said idea that Vergil wants to overthrow Mundus and take his place so he can run the world and turn into his personal utopia, because he believes the world would be a better place if he has the power to protect it (which could make sense if they borrowed classic Vergil's guilt of not being able to save his mother when he was a kid), and by protect, of course, he means control it. Obviously, he doesn't say this to anyone, specially his own followers, because what they want is freedom from the demonic rulle, not a new ruler. He looks at his allies, including Dante and Kat, like mere tools for his ultimate goal, but keeps a friendly and sympathetic facade to gain their trust.
Or, alternatively, Vergil is really a nice guy from the start and he really wants to defeat the demons, but once he starts to gain access to the demons' power, he becomes corrupted by it and turns on humanity to become a new ruler.
Whichever scenario is likely (and not just the two I mentioned), I really doubt the members of the Order themselves have any mystery behind them, except maybe if part of the story involves one member being an undercover demon working for Mundus.