As long as you aren't doing any magic projects on your phone, pretty much any usb port will connect the GNex to your pc. The phone has to be on of course, connect it to your pc, XP automatically recognizes it. I rarely use Windows Explorer, I use Power Desk as it has so many features, but in this instance PD doesn't recognize the GNex where as Windows Explorer does.
that's weird, I've always been able to copy to folders, although I don't download music....there's a feature in the settings->developer, then choose USB Debugging. If that does't fix it..I don't have iTunes app installed, what specific music app do you have on your GNex? I'll download it.
I just found an app called Double Twist, it has sync w/ your mac or pc, I believe that would be a solution. The only way I know of truly mounting the internal sdcard is to root your phone, but it ruins your warranty plus it's totally unnecessary, try the double twist app.
GTunes Music Downloader V6 has options to share, do a search on G Play sync music iSyncr has a free option, plus a paid option w/ the best rating. I'm trying the free option now
Most of what I saw, the syncing to your pc is a paid ap, iSyncr Lite is free, you download an app to your pc....from there you have an option of using usb or wifi.....definitely looks like a solution for you.