I KNOW THIS LOOKS FREAKIN LONG BUT IF NOTHING HAS WORKED FOR YOU IN THE PAST 65 PAGES, THIS MIGHT HELP!
The problem is with iOS7 I think. I haven't had any problems with my iPhone 4/iOS5/iTunes11 combo until I updated to iOS7 a couple of weeks ago. Since then I've been having problems adding new music to my iPhone. Even when I unselect sync music, it's still all present on my phone.
I think the problem is when you update to iOS7 and have music on your phone, it consolidates it to the phone's memory and makes it untouchable. Then when you try and sync, the phone tries to sync it all over again. Hence why you see duplicated and have problems with playlists. Also this is why a full reset of your phone works - you're installing iOS7 with essentially a wiped memory.
My fix was to place all music I wanted to add to the phone in a separate playlist and only sync that playlist, nothing else. All my music from before I updated to iOS7 is usable on my phone, but in addition, so is the new music I just added.
When I tried to add an older playlist, all of the songs from that playlist are now duplicated on my phone (because they were already there before and that music is now permanently stuck on the phone + it gets re-added)
When I tried to sync as normal, it was telling me there was insufficient space on my phone (the bar shows 8Gb free). I think again this is because the music on your phone pre update is visible/playable but not affected by syncing and it's trying to add them all over again. For some people the other hotfixes may work because they have used less than half their memory and hence the phone will have sufficient space to add all of the songs for a second time.
This explains why some fixes work for some and not others, why a factory reset is the only true fix, and why you're able to add certain sized playlists and not others (and why you can only add songs via playlists now)
In conclusion, my fix:
-You have to manually manage music. Untick all boxes.
-Put all music your from after iOS7 update you want to add to your phone into a playlist
-Only select that playlist to sync.
-In your phone you should see all your old music + anything from that new playlist
Sorry for the long post, hope it helps at least someone else