so we can assume that kogasa was turned into a rottong corpse
Eh, I think there is pretty decent support in both canon* and fandom that Kogasa is the umbrella (or if you want to be pedantic, the spirit birthed from and currently inhabiting the umbrella). The human-ish body is just to provide better dexterity and mobility, and a "form you are comfortable with". Seems to be a thing mature tsukumogami can do in Touhou.
So if this is the case, Kogasa is just fine; she is right there.
But yeah, what happened to that body? O_o
* Sure, ZUN never confirmed or denied it directly, but from some of the dialog in the games plus what Akyuu has written about tsukumogami seem to give good evidence.
