Hey sakendrick, for the Zune DRM files have you experimented with the WP8 for WP7 desktop app? There's a chance this app might sync your DRM music where WMP currently doesn't. Is it possible that out of your 300 test songs only 55 were not DRM?
I *believe* I have solved my problems... need to test but it turns out the issue was with my playlists. Since some songs were syncing and others weren't I took a look at the playlist to find out if there was anything I could see that would distinguish the songs that did vs. those that didn't. I opened the *.zpl or .wpl files in notepad. Turns out there was a weird relative path for many of the songs that started with zunemp:// where as some had normal c:\folder\folder\song paths. The numbers didn't line up 100% but they were close enough and on spot check - those that did sync had the c: type path. I then checked my other PC where the playlist would sync 0 songs. Every song in those playlists had bogus paths to an F drive that no longer existed. The weird thing is that Zune some how compensated for all of this. Those playlists with the F: would play fine in zune.
So I did a simple CTRL H (find and replace) in note pad and insert the correct path to where the songs were stored..Presto! seems to have worked.
I've been doing a bit of experimenting because the song duplication issue is really annoying for me (I always listen to whole albums, so song duplication is a deal breaker). I've come across some interesting behaviour, and (I think) a consistent way to stop it from happening.
Some background first: I have a Lumia 520 and keep all of my music on a 32GB microSD card. I sync just using drag-n-drop/cut-n-paste in Windows 7 x64. All of my songs are in mp3 format (a lot of albums are in FLAC, but I convert them manually beforehand) and I keep all of the tagging neat, tidy and consistent using mp3tag. The folder structure of my collection is Artist\Album\Track - Title.mp3. All of the Xbox Music stuff it turned off so it doesn't mess up any of my hard work.
I think the problem occurs when you try to sync too many songs at once, like dumping your entire music library onto your phone at once. My collection is only 696 songs/7.4GB and it almost always produces duplicates when I transfer all of it, regardless of whether I format the SD card beforehand or not. When I was testing out different things trying to find the cause (e.g. playing with tags, doing things in a different order, etc.) I started transferring smaller numbers of songs so I didn't waste 30 minutes at a time to see if something worked.
I'm not sure if it has to do with the number or the physical size of the files you transfer or even the number of folders, but I found that by syncing less than about 120 songs or 1.20GB at a time, duplication wouldn't occur. Whenever I did get duplicates, I deleted the files and the offending albums from the hidden Music\Albums folder on the Phone storage (not the SD card), then I disconnected the phone from USB and checked that they were indeed deleted in Music+Videos. Redoing the transfer but with less songs worked every time.
TL;DR only transfer around 120 songs/1.20GB at a time (the problem may be number of files or total size, I don't know). It's a painstaking process, but it worked for me. Maybe not so feasible for those with big libraries though.