Just a quick update on my end; I have slowly and painfully chipped away at my 18,300 songs, and am currently at about 2200 left to go. I had a day where I was doing a lot of relatively mindless tasks at my desk, so I called in a support case while I was repetitively launching sync attempts, and have progressed on the case to where I will be speaking with an engineer tomorrow as the support manager saw what was happening and escalated it accordingly.
It was literally dozens of sync attempts yesterday, with my best/most productive getting something like 1150 songs synced, and the mean was around 200 songs at a time. You can do the math, I'm just tired of it. :-s
Basically, it seems to time out/reach some sort of data transfer limit and just completely hiccups out of the application. No error message, no status code, and Apple Devices is no longer visible or traceable on my PC.
Anyway, if any random Apple engineers are checking in on this, it is not a setup issue, it is not a network issue, it is a bug. You should probably try to fix it. (And thanks to the support staff who moved my request fairly painlessly and quickly through their requisite layers of checks and balances.)