"If I hit the yes button, Synaptic just vaporizes with no error message."
you can't hit yes, when this message appears, it means that part of the files are not there and the upgrade needs them so it won't work, you can end with a useless desktop if it works, cancel is what you need to press
you could try to use another repo and try again
i couldn't follow those steps, what i did in my two machines was remove kde4 altogether, including some packages that couldn't be upgraded, then i logged out and from console i called synaptic and installed all kde4.8.3 packages i considered necesary, it worked on both machines
i think i made that process because kdeupgrade didn't worked for me and direct upgrade, what you did, didn't worked either, this is the most complicated way i think but as i said worked two times