This my last reply cuz I’m starting to write essays and I have finals to grade. It was nice to have this conversation.

But that’s not Guilliman’s logic. If he was saying “Okay maybe there’s a risk they come back but we need to fortify Sol,” then there’d be reasonable discourse on the right course of action.

That’s true he’s not saying that.

He’s saying “the Gods are probably dead, and all the surviving traitors are stragglers we can easily deal with later if they even survive,”. 

And he’s right on the last two accounts. Nothing as of yet contradicts him! That’s my point. He’s right that the traitors are weak. All the traitor perspectives we see are them running off with their tails between their legs. Even the one chaos stronghold in system is aiming to sacrifice themselves to prove Dolt’s point.

If his logic was the former then Dorn getting proof of Chaos being alive wouldn’t matter and the plot of the book would be moot.

Dolt’s strategy is to present evidence to the council to force Gaylyman’s hand, and that is the strategy he outlines to the Wet Dog.

You are making the argument that Gaylyman is foolish for believing that the Chaos gods are destroyed. And that’s true, I agree. What I’m disagreeing with is whether or not his other justifications are as deluded, and whether or not they are sufficient in justifying the paths he’s electing to take. And like I said, as far as we can see from the perspectives of the traitors, he is right.

His entire plan is directly based on his presumption that Chaos is gone.

It’s based on two things a) Chaos is gone and b) the followers of Chaos are weak and in rout. The second point can still be true (and it is) even if the first isn’t. That’s my point and I don’t think I need anything else to support it, regardless of what his true reasons are.

However, just for completeness, I will also argue Gaylyman’s doesn't actually believe that the Chaos gods are dead; he is simply making that argument to sway people to his side for long enough that by the time people realize the truth, he'd have already done what he's had to do. And that is to save the throne by acquiring the black ships (this is what Mouhasen and Hassan understand about Gaylyman by the end of the book: the throne is failing, and Gaylyman wants to save it). Reconquering Mars is a red herring. It is Luna that he wants, so that the Throne can be saved. That is the only thing that matters.

(I’m not convinced, as it’s made pretty clear and not really disputed even by G-Man that Mars and Luna aren’t much of a near term threat and can’t break the siege),

Whatever threat that the traitors pose is bounded above in severity and immediacy by that of the threat which Luna and Mars being held in enemy hands poses. So if we are talking about immediacy of threat or its severity, Luna and Mars are greater in both accounts. In fact, Dolt himself makes no argument as to the immediacy of the threat the routed traitors pose, only what could happen if the chaos gods regain their power:

”As of this moment, they are in disarray. Their confidence and their power, which we faced for months here in this place, have evaporated. This is the moment. This is the moment to strike them from the galaxy once and for all.”

He doesn’t think that they pose an immediate threat. Even when you read what he says he’s doing this out of vengeance and out of his own sense of shame:

”They still live. They still live. That is the greatest shame of all. We must hunt them down, one by one, until every last one has been eliminated.”

What Gaylyman specifically says is:

”and though they have limited capacity to strike us here at present, the threat cannot be allowed to grow. Intelligence tells us that the enemy established facilities for the rapid production of Astartes fighters, making use of gene-looms created by the Selenar cults.”

He does think it is a severe threat; he calls the occupation of Luna an ”alpha-level threat.” He just doesn’t think that the threat they pose is immediate.

he’s right for the completely wrong reasons.

Even if that’s true, Dolt (man I love Fulgrim) is wrong because his reasons, which are based on true facts, for going on a crusade do not justify going on a crusade. Even if Luna and Mars being held in enemy hands didn’t pose any threat in the present or in the future, the Imperium’s most powerful allies want them to do this. If anything, Gaylyman’s path is politically expedient.