I just put a solid 25 hours into a playthrough of TLOU, granted i'm an achievement hunter so was looking for all notes, comics, pendants, shiv doors, safes, etc AND was playing on Grounded.
I think a lot of what you are saying sounds about right, but I do have a prediction that the game will leave it off almost like The Sopranos, almost fading to black with us never truly finding out if a cure was made. This doesn't seem to be the type of franchise that would end things in that straightforward of a manner, but I may be wrong.