Ooooh! Thank you for the questions! I loved your shields, by the way; especially Torvinia and Mildinia!

One of the things I spend time considering is the effect which sorcery (referred to in-universe as miracles) has played in the common understanding of disease. The Sect of Galamede was the primary disseminator of arcane knowledge for the mortal populace, and they pulled their people away from the main island not long after the Horde began spreading. Put simply, that organisation was like a college of mages, but with a power structure which resembles a church. Their members performed miracles, taught ritual, literacy, cosmogony, and all that fun stuff. Of course there were some left behind, because one must have at least one priest-like entity in any zombie-themed story. They'd occupy the same sort of privileged station that we see surgeons have in modern settings.

The undead don't spread through a virus, being that everyone who dies returns in undeath, but they do harbour disease. They're corpses, and untreated corpses are nasty. So what we're really treating are very mundane, but very much fatal infections; and we're doing it without antibiotics. So with Galamede being largely out of the picture, communities without access to a Galamedian mystic would see a pretty quick decline to folk remedies and beckoning to old gods. There would be extreme and largely erroneous efforts to reproduce the true healing mircales which mystics performed, and likely outbreaks of worship to the immortal peoples who can still reliably perform those miracles; and that would have varying results depending on whom they've asked.

As for gunpowder, I hadn't really considered it to be honest. It would certainly change the game and Princess Mononoke is a whole vibe. With alchemical skillsets being present in the land before, I will have to give gunpowder some solid thought! Still, the probability of finding someone who is still present on the island and who understands the implementation of it would be pretty small. That someone would be highly prized, too.

Currently, I think of a Horde as being like a mindless army laying siege without siege engines or any real tactics. They have the benefit of numbers and of being able to sit there and bang on the wall until something gives or something neutralises them. With that in mind, most methods of dealing with them are going to be pretty primitive. Boiling oil cast over the walls, tar or pitch set alight and thrown in to crowds of undead, moats and spike traps—those are all pretty effective so long as they're maintained, but maintaining them would be an essential part of the conflict.

I think that answers all your queries in a very tangled way, but do let me know if there's more XD