Nicodemus is not a Fallen. We are talking about him, not them. So "their" and "they" don't apply. The Fallen doesn't necessarily wish them to lie. We don't really know what the Fallen want apart from corrupting mortals. We don't know why. It's never been addressed. And you can see in my post above and in Snark Knight's what our theories are about they.
Harry's plan in Skin Game was based off the fact he and the Denarians are allies of convenience, but not friends. Harry was forced to work with them and he has a highly antagonistic relationship with them. If he didn't I doubt they would have betrayed him as they want him on side. Recruiting is what they do. Much the same in Small Favor, the main difference is that Harry is actively working against them. Both times Harry is betting they underestimate him.
Not sure how me believing Nicodemus will lose me my very own soul. I assume you are being hypothetical and using the royal "we"? But again, it's about Nicodemus not about the Fallen.
Actually, that negates his choice and shows you don't understand one of the central tenets of the series. Nicodemus made his own choice. Anduriel did not make him do anything. Because in the Dresden Files, no being can make a mortal choose to do something. Nicodemus chose to kill his daughter. I agree that he lies to himself. And that he probably lies more than some humans. Not more than most. He isn't a compulsive liar. Most of what he says is true. His default mode of operation (as you put it) is doing whatever it takes to achieve his mission. Lying is one of those things. But so does every character. The only characters who cannot (unless compromised by magic) are the Fae. The Angels seem to have enough choice in terms of choosing when to lie. Which makes interactions with them FAR more dangerous because of their incredible knowledge and perspective. I don't argue that he uses truth to support more powerful lies. But you would have it that he lies constantly and compulsively. He doesn't lie in every speech or interaction, and you are still yet to provide even a shred of text to support your claim.
I think you don't understand the phrase your using, or what my argument was. So I will endevour to be clear.
You said that "If it's too good to be true, then it probably isn't true" and meant that in terms of when a known "con man" is talking to you.
I am saying if you are unaware that the con man is in fact a con man, why does that mean that whatever they are telling you is more likely to be true? Why is their truthfulness dependent on whether you know they are a con man or not? You're argument doesn't make sense.
I have read the books and am perfectly aware of how the Fallen operate - based on Harry's encounters and perspective. But how they operate is irrelevant to your point about when to trust a known or unknown con artist. Not to mention, you keep conflating the Fallen with the Denarians. A Denarian is the combination of the Fallen in the Coin and the human bearer. A Fallen is the Fallen Angel that no longer resides in Heaven. The Fallen, and indeed Denarians, may run cons. But Nicodemus is not a Fallen. He is the human part of the Denarian, which is a title and an office. Not a being. Nicodemus doesn't go round pretending to be different people and running various scams. He doesn't need to. He introduces himself as Nicodemus, and does whatever he has to to get what he wants. He might lie or trick from time to time, but he isn't what is considered a con artist, which is a type of professional scammer. It isn't his profession or his office. He is a leader of a a pretty nasty group that is trying to cause chaos and misery, also while saving the world (whatever that means to them).
Pure speculation again. Why would Nicodemus stop helping Harry with the Reds? Once he finished with them I imagine he would try and convince Harry to help him with his longer game. But as you can see he doesn't force any of the Denarians to stay with him. Tessa and her lot choose to do other things. Choice is important to a bunch of beings that felt they were denied it. Lash makes that very clear. And why would any of that lead Dresden to killing his daughter, just because Nicodemus did? Where are you getting that? In no way has it ever been implied that Harry would end up murdering his child.
He goes to Mab because he thinks that's the easiest one to get out of. He commits suicide immediately after. If he had chosen the Denarians I think he would have still killed the Reds, Jim says he would have been successful whatever option he took. The main difference would be what the series would have become. Let alone Harry the Necromancer or Harry the Necrogod. What Harry tells Mab is partly true. But he omitted his plan to kill himself (as he had wiped his memory) and was unaware of the Fallen's influence in pushing that plan. He might have actually chosen something else if the Fallen hadn't tried to remove him.
I have never said Nicodemus' main goal is the Reds. But it is a goal to remove them, at some point. He actually tells his reasons to Dresden. My other thread that I referenced in my previous post discusses those reasons and the why. But to summarise, for someone who is trying to "save the world" or perhaps end it, the Red Court are a major problem in the long term as they likely didn't want to be ruled by Nicodemus (if that was a goal) or destroyed like the rest of the universe (if that was his goal). And how do you know that his main goal is Harry's soul? Why do you even think he cares? From what we have in the series and WOJ, it seems his end goals involve the destruction of humanity and perhaps the universe.
The Fallen may have wanted to Fall but we don't have much to go on. What is clear is that they found it abhorrent to serve TWG and have his Will and rules govern them. Lash tells Harry this early on. Not only that, but as Snark Knight and I were discussing above it is possible that the Fallen may have an issue that mortals have Free Will and the danger that poses to Reality and by extension, themselves. They don't want the Old Ones and the Outsiders to end the universe. Except for perhaps certain rebel Fallen who just want to burn the house down. But Hell quite possibly seem humanity as a massive security risk.
Nicodemus doesn't fight for souls. The Fallen do. The Angels do. Nicodemus just wants Harry on his team rather than against him. But even he is very prepared to kill Harry if necessary. Nicodemus isn't the Devil. Don't confuse the two.
The Devil cares about souls. But this is because, as Jim puts it, "he has a argument with GOD". The second part of that quote also says that in order to have an argument you have to have a universe (which puts limits on the Devil) and so in some ways, Nicodemus is more dangerous to the universe than the Devil. Which implies all sorts of bad things.
Nicodemus doesn't have to care about his promises. But if he wanted Harry to work for him, he is more likely to honour those promises. And he DID want Harry to work for him. I quite suspect if Harry were more agreeable he would find Nicodemus a very different type of colleague. Which doesn't make him good.
Indeed, I suspect it would be easier for Nicodemus to convince Harry if he knew him better. It might even be easier if Dresden knew Nicodemus' soul (depending on what's in there). Marcone has convinced Harry that he is worth working with from time to time because there is some good still in him, and because he is better than the alternative (which is chaos). Harry isn't stupid though and knows eventually Marcone will become too great a threat. Regardless, it would be to Nicodemus' benefit to honour his promises to Harry about the Reds both because the Reds were a problem to his long-term plans and because he wanted Dresden's loyalty. And Dresden as an ally is a very powerful weapon to have. As many, many characters have realised.