Here are a few ideas for some wild card events to complicate things, perhaps even turning the whole affair into a wider World Core War I. Sorry for the length of this post.
1) Mother Fury (in canon currently residing in Kartakass, details in CoTN: Werebeasts p. 67) and her clan get chased out by the native wolfweres that dominate that domain. She and her minions make their way north through the core fighting (and losing) against the native defenses, but with each conflict Mother Fury becomes more cunning and insightful of the ways of man. In Falknovia, she cuts a swath through the Falknovian soldiers and police forces before coming into contact with Vjorn Horstman (CoTN: Werebeasts p. 78), who is duly impressed by the power of her clan. Horstman, in turn, impresses her capabilities to the high ups in Falknovia's military and intelligence communities.
Horstman approaches Mother Fury for an alliance, and it's a match made in hell. Horstman has a lot of reason to ally with her- to study her kin more directly to distill a better brand of beast-man. Mother Fury is also interested in embracing the human half of her kind as another kind of predatory exemplars of the Wolf God (referenced in a sidebar in Gaz 1, p. 88). The former can give the latter new recruits, and the latter can give the former new test subjects. Eventually, as the war rolls around, the Falknovian intelligence operations discover the rumors of werewolf faithful to the Wolf God located in the Forest of Shadows. Mother Fury is offered a portion of it if she can capture the area and "convert" the werewolves already there. As lycanthropy is normally incompatible with undead, Fury agrees. If you play fast and loose with "the only one curse at a time" rule, and rule that the Creeping Death is a curse- then you might be able to justify lycanthropes being immune to turning undead.
2) Fiends and reality wrinkles to get around the barriers. You have a few options here. Martira Bay has one notable enemy of Azalin- Styrix the Night Hag. Her goal is to gather all that soul energy- perhaps she might be motivated to assist the Falknovians in subverting the town in return for some sacrifices. Most notably, those ground into her device might not produce bodies that could be reanimated by Azalin. Another option might be the Chauteaufaux fiends, who in canon were slain inhabiting that Dementlieu town. Perhaps they were simply driven out, or as mentioned here:
http://fraternityofshadows.com/wiki/The_nameless_Amnizu Perhaps captured by the Fraternity of Shadows. Either way they might be motivated to work with the Falknovians- as revenge against an ally of Demenlieu or turned to work for the Falknovians as part of the Frat's efforts to unseat Azalin. One last option might be the return of the Whispering Fiend, who seems to strive on the chaos and bloodshed brought about by the war. He's likely to be more of a wildcard, but he might destroy a settlement or two of Darkon at a pivotal moment.
3) Ezra’s Holy War Straining the Treaty of the Four Towers- Raines may decide to seize the opportunity and declare holy war on Azalin and his army of the undead, calling upon the other branches to join him. This puts the Home Faith in a bind- as its relations with Boristi are already tenuous, and she's not fond of Falknovia. This latest internal tension might light a powder keg and cause strife within the church about not only what to do about Azalin but also what to do with the murderous woman who would ally with him. This trouble in Borca could effectively keep it from participating in a counter offensive against Falknovia. Meanwhile, what effect such a call might have on the Mordent branch, traditionally at odds with Darkon branch (I think either Gaz III or Heroes of Light talks about the Mordent branch planning a secret crusade against the Darkonese branch), remains to be seen. In any case, perhaps the Mordent Branch would use this as an opportunity for an excuse to infiltrate Darkon. As, of course, as the Ezra crusaders who decide to go to Darkon stream across Mordent, Lord Godefroy could begin killing them and adding them to his collection. (Godefroy in Gaz III is described as becoming more active and almost wanting a crusade to take place in Mordent- this could be where he gets his start.)
4) Lamordia- The Syndicate of Enlightened Citizens (Gaz II, p. 79) is noted to have secret influence everywhere but is notably focused in Lamordia. If Azalin was revealed to be a lich (a spellcaster in power is bad enough), I think they could really agitate for Lamordia’s entry into the war, or at least support of Falknovia. They have at least some sway in Darkon’s clergy and could conceivably have moles planted in various other places of power within Darkonese society
5) The resistance movements in Falknovia- Just to balance things out and give a few opportunities for those who would act against Falknovia, perhaps it would be wise to take into consideration what effect Falknovia's own resistance movements might have against Falknovia. With much of its troops at war abroad, on the home front things are likely to be weaker. Gondegal, the Knights of the Shadows, and even the kobolds (led by the Spawn of the Lizard and the Basilisk) are all noted enemies of Falknovia's oppressive government. IIRC in canon the various Falknovian rebel forces have never gotten their act together to really cripple Drakov’s military machine. Perhaps these circumstances (along with player intervention?) could galvanize them to cause Falknovia enough internal damage to let off the pressure in Darkon. Alternatively, if you wanted to discount the intervention of the Knights of the Shadows, you might have them become involved in the events related to Ebonbane and the maintenance of his seals (this was discussed in one of the framing fictions in one of the BoS’) or other events in the Shadowlands cluster/domains that would later become the Shadowlands cluster.
So yeah, basically, given the right circumstances, you could have a loose coalition of all sorts of tiny conspiracies serving interests normally at odds with each other aligning against Darkon. Ezra firebrands, rationalist manipulators, Wolf God lunatics, and horrible fiends all give you some variety in foes a Darkonese defense force might work against. A game could be made of exposing them to each other and then putting them at cross-purposes once again.