We don't know how his Commandment works for now but I am not saying that he is lying, I am saying he has his own agenda and revealing stuff about Gowther makes it harder to kill him, since killing him would awaken Gowther, who is more dangerous than him. If he only wanted to put them against the Sins I don't see why he revealed what would happen if he died. He did that to dissuade them to try killing him. Among all the things he's doing, I find his attachment to Dreyfus's body shows that he isn't exactly forbidden to keep doing what he desires but that's only if we take the strict definition. As I said we don't know how his Commandment works for now.
I think it's pretty straightforward. So far all the commandments were that.Unless we assume that he is a compulsory liar, which again I really don't think it's the case.
Fraudrin prolly didn't have any reason to lie about Gray Lord at least.Because if the latter is a pacifist and can't kill, then hiding the truth from Denzel and Deathpierce would have them fall for Gray Lord's commandment. If they killed Fraudrin inside Dreyfus body, since Gray Lord is "Thou shall not kill" they will be affected by his commandment.So why reveal it? He could also leave Dreyfus body if he wants to fight without restriction.Leaving the attachement he has for Dreyfus aside, and maybe him possibly needing it for something, I think it's plain obvious why he revealed the truth about Gray lord. He is the Selfless.
What that entails is that he can't do anything selfish, just like Galan couldn't lie or go back on his word, and just like Gray Lord can't kill apparently. So him keeping that information to himself can be seen as selfish. If he did it's likely he would break his commandment.Just because something hasn't been explained means we can't deduce it based on previous examples.So he merely shared that information .Because in my eyes
1-he can leave Dreyfus' body if it's really necessary, and a life or death situation is just that. So if he's threatened he can and should leave his body.
2-no matter how strong Denzel might be and well we know Deathpierce's PL, they can't be as strong as two commandments. Even without their magic they should be very strong.And more importantly Fraudrin still has his magic because he avoided the seal.
So based on that we can deduce that he prolly didn't have much of a choice, since keeping that information for him
self
can be seen as something
selfish.Also you seem to forget that Denzel was hiding something, or someone. That is why Fraudrin prolly wanted to end the fight, rather than it happening, because Denzel still had his trump card(the woman in the sword). Gray Lord already hinted at that, so as long as Denzel was hiding it they simply couldn't take the risk of a real fight happening.
And I already explained why he revealed the truth about Gowther.The Pleiades going after him simply means that the sins will retaliate. Mel seems to know about Gowther, at least it was foreshadowed back whe Gowther went berserk. So he is prepared to that possibility, and even considers Gowther his friend.So if the latter gets attacked, no matter who it is, do you think Mel will side with Denzel, or his friend Gowther?

He prolly already knows the latter is a demon, and Mel is a demon himself. So to him race isn't what matters, it's the individual.
So again Fraudrin's plan was to pit the Pleiades against the sins, because Mel won't give up on Gowther.
So no Fraudrin doesn't have his own agenda like you say, everything he's done up till now was to help the demons. Let's think about it. For some reason he went to Danafor and killed Liz, which is strongly hinted to have been reincarnated as Elizabeth.I don't think it's a coincidence he attacked her, it's either because of Mel, or because she was the previous apostle of the Goddess.So he needed her blood to free the demons, just like Elizabeth. While he did avoid getting trapped like the other demons, he actively tried to free his brethren, by possessing Dreyfus. While in Dreyfus body he could have lived as a human if he wanted to, without so much lifting a finger to help free the demons. So if anything that shows that he's not being selfish, in other words he's selfless.He has to or he'll end up just like Gowther.