Love the idea of Armaggedon, simply because it does open so many possibilities, and can make for some interesting massive battles. The scenery in a full, all out war across the multiverse seems quiet intreging.
The Sigil element is something I personally don't agree with. I don't claim to be an expert on such things, but as I understand it, the Laws in Sigil function as an extension of the Lady's will, rather than any inherent law. If she is who watches the Gates and bars Gods from entry than making her a God using worship would be pointless. She has been worshiped before, and it did little more than provoke her ire. Of course, given the differing theories on what the Lady is, and why Sigil is her domain, your idea has various complications.
Theories
1. Sigil is referred to as the Cage by most. What if the Cage is not keeping her out, but rather what is keeping her in? That Gods and Archdevils cannot enter Sigil not because she keeps them out but because they must keep her in.
Forcing her out of Sigil would be something akin to letting free a trapped Tiger or an insane killer.
2. She is God-like, much as the Archdevils, and other such creatures, and thusly holds Sigil as her domain. As anyone can tell you, it is difficult, if not impossible to assault a God in her domain.
3. She is Something Else. Perhaps she has gone beyond God hood to something else and condemns worship because she feels it demeans her.
4. She is the Prime (First of) the Romani,existing as a proto-god and ruler of the city. Perhaps she is why their magic works upon the spire. And should she be ejected from the City by "the Rules", than she would return to her people and her power base. And they fight dirty.
As shemeska has mentioned before me, I must note that she can eject people from Sigil, mazing or flaying them, and can close her portals beyond all reproach. Also note that only demons & devils below Pit Fiend and Balor level can even enter the city of doors, just as Archdevils and Lords of the Nine are barred. So the war would lose its most powerful fighters, adding to this is the fact that Sigil portals lead just about everywhere. It is almost impossible to even think about holding a place where you can be attacked at all sides. A concerted attack by a force even a portion of their size able to attack from any angle at any time would demoralize and destroy them. Just ask the VietCong.
Since they would be unable to close those portals (Sigils portals cannot be sealed), they would be stuck in a place with large army deprived of its generals and vunerable to attack from all sides. It would get ugly quickly.
