One thing just came to mind, as I read back through the last few pages. Well, a few things, really.
1) Things can be dropped (and yellow dwagons can crap) offturn, and cross zones that way
2) Being dismounted/having your mount shot out from under you counts as a "fall" which can kill you, but can let you cross zones (see wanda falling from air to ground when her mount is disintegrated by archons, ansom falling from air to wall on being dismounted)
3) The whole "quick enough to catch us when we fall" thing (I may be misquoting, but it's close enough)
This may have been suggested for some previous update, but how 'bout this?
Wanda has her mount shot out from under her (or really, waits until any dwagon is shot down), then falls, decrypts the fallen dwagon as it hits the ground, and then lands on it. We know that you can jump/fall and land on a mount without injury/damage - Jillian does it on several occasions - so this might allow Wanda (at least) to cross zones, allowing her to start building up a decrypted army on the ground. And if this is disguised by Jack's foolamancy, then Jetstone will still focus on trying to shoot down the dwagon(s) that appear to carry Wanda, assuming that she must be up there (because killing her is their greatest priority).
Add that to a portal-based incursion from Parson and co. (yes, I'm on the side of the portal-based attack theory, since MK/portal has been foreshadowed as a future part of war in Erfworld) and you have a small, but powerful force on the ground, consisting of at least one warlord (who is low-level, but very large, presumably correspondingly strong - he may not think of himself as strong, but with a twoll-sized body he's gotta have more muscle than the average erfworlder, and thus more power behind his swings - not counting his other potential abilities), and three powerful casters who, it should be noted, have already shown themselves capable of destroying a city single-handedly. Plus, of course, any enemy that Wanda decrypts, any of the air force that fall and get decrypted, and the incredible bonuses of a) surprise and b) potentially ambushing Slately. After all, Parson has considered the Ender's Game gambit in the past...
It's a weird combination of theories, but feel free to play with it, ignore it, denigrate it, whatever.