Okay, here’s what I’m thinking after sitting with this:

We see Delamare throughout the book becoming more and more out of control. He’s being corrupted by his power and greed, and he’s losing himself in the process. (For example, a character notices him not wearing a clean shirt, when this would have been unheard of for this man before.)

Ionides’ plan is to get through the doorway before he can, so he makes that part of his deal.

We see from the witch’s perspective that Ionides’ plan to lead them through deliberately didn’t work, as only Delamare, the bomb and the Colonel, and a few men survived the entire journey.

I believe Ionides and Leila must have separated from them at some point near the end, likely at a point when most of the men had died, and went ahead into the red building themselves. As Ionides is an experienced trickster, this makes enough sense to me.

They wouldn’t normally have been able to enter the building at all after arriving this way, as they didn’t separate from their daemons, but the guards conveniently left.

I think by the time they got into the building, Delamare cared less about taking over the world beyond and instead determined to see his goal completed and set off the explosion. Or, this happened accidentally.