Heen was pretty much the only realm that didn't make an arrangement of some type with any of the invaders.
I do wonder what might have been had Mesh gone all the way and made an alliance with the Monsters. Instead, she had a short dalliance with the idea and then rejected it as she did not wish to sacrifice her populace. She wanted Monster help without offering them any benefits. To be fair, if I recall rightly they did ask for Mesh to abandon her democratic heritage for a single reliable King the Monsters could work with during invasion, but in the face of annihilation, was it so unreasonable?
At any rate, Mesh couldn't bring herself to support the Monsters proposal. The small contingent of monsters that had gathered around Twillen and the monster lord that brought them disappeared even as the daimons advanced. I think it's interesting that part of the decision, aside from appointing a King, rested on the loss of strength that would go with sacrificing half of Mesh' populace to the monsters, but rejecting monster alliance is essentially what condemned the
entire populace of Mesh to blight and destruction.
Did so swiftly swallowing up Heen and Mesh essentially deliver an unassailable daimonic dominance over other factions, human or otherwise? All the factions seem to gain strength from taking human regions and bleeding the local populace dry and almost from the outbreak of Invasion, the Netherworld had most of Western Beluaterra under its thumb. Due, almost entirely, you could argue, to the intransigence and short-sightedness of Heen and Mesh in their respective approaches to the Invasion.