Ah, a material. Useful things, these.

You know, sometimes I forget that me and my roommate got a green balloon and painted trahearnes face on it

Many criticisms of common trends in Japanese media, from the reactionary nationalist sentiments to the sexualisation of minors, are true enough. But you so often see them presented in a myopic way, with Westerners talking as though their country is any better in this regard when they're very clearly not. Like it's strange to see a US liberal talk about how Japan "hasn't come to terms it's Imperial Atrocities" as though Japan isn't covered in US military bases whose occupants abuse the local population with impunity, or complaining about Japanese age of consent laws when their nation still largely allows child marriage (as young as 15 in some cases). Now I'm not saying it's wrong to make these sorts of criticisms, as long as you do so from a place of actual knowledge and understanding rather than a vague jumble of stereotypes and "common knowledge", but you can't be detaching them from their broader context and treating them as exceptional when they are actually rather mundane.

from what I've seen of the plague's final stage sequence, perhaps the symptoms are a result of their physical form literally just deteriorating as they get closer to the transformation and as that power becomes the only thing that sustains them

My theory is that dragonsplague is similar to bestowal of spirit, where the pawn starts getting a will of their own and becoming more "human" (all of the observations mentioned above could support this theory). However, the process seems to be much more chaotic and quick, while also being different to what happened at the end of DD1. Unable to really control this fledgling will, they explode into a writhing, draconic form, except in the true ending where they gain some sense of control thanks to us.

Instead of changing their appearance to the Arisen (which seems to still be a lore thing according to one DD2 loading screen), it seems they are "returning to oblivion" where, if Pathfinder's to be believed, all pawns come from. Since "oblivion" was given form and purpose in the form of the dragon (Nex and subsequent dragons), the dragon form our pawn takes echoes its alleged origins. It could also be why they transform when Nex appears; perhaps they were compelled to "return to oblivion," but their relationship with the Arisen prevented that.