During the invasion of "Mutare's land" (Nighon) in map 8, there are no tunnels leading into Erathian territory anywhere. Just another hint I guess.
Erathia was being simultaneously bombarded by five-six kingdoms, including powerful aliens from outer space. I think it is fair to say this is not a standard or fair example. Deyja didn't make any move before King Gryphonheart's reanimation so I don't grasp the point.(After all, Nighon succeeded in taking over Steadwick, and Deyja didn't), and they couldn't have done so unless they were better organized and/or had a stronger army than Deyja did.
Why should it take a ruler and no internal squabbling to properly organise Nighon into an offensive force when there's literally no mention of any active ruler at all in H3? Day-to-day life in Nighon is barely organised at all even with a strong king, as we see constantly in The Sword of Frost. Dungeon Overlords launch raiding parties for land. If they are competitive, killing one another and trying to become king, logically they will want all the land they can get. Erathia is on its knees. They cooperate with Sandro and attack Erathia for more land. Where's the inconsistency here?
Note that Nighon, like Deyja, actually did lose the war in spectacularly miserable fashion - attacked through their own tunnel network - and mostly succeeded thanks to their big alliance, the massive international assault on Erathia and the element of surprise to begin with. The size of their army and its level of organisation is pretty much a moot point.
If you think the Queen and the Mothers are the same beings yet your point is still that the Restoration War could not have happened after Mutare's death, you just defeated it utterly since the Gold Dragon Queen was quite brutally killed by the Kreegans at the very beginning of the war.