OK, if I read this right, you have 6,500 songs in your iTunes library. Most of those songs either matched or were uploaded. You still have 600 songs that have not either matched or uploaded, is this correct? You've enabled iTunes status in the view options and don't see any "ineligible" files, correct? These are all music files, correct? Not all media files are eligible for iTunes match.
Regarding hangs while uploading, others have had this problem, but I haven't seen any clear solution. Some have theorized it may be more a matter of the internet connection, that even brief interruptions in the connection could stall things and iTunes Match won't re-engage, or that heavy bandwidth use is causing some people to experience first time their ISP throttling back their account.
Regarding your match vs upload experience, honestly there are a lot of people who believe they have songs that are also available for sale in the iTunes store, yet they don't match. (note if they were once for sale, but aren't there now, they will upload, not match). While some people have found something unique to their situation that they are able to fix, nobody has an explanation for situations where, say, a CD was ripped by iTunes and all the songs save one, get matched. We're all hoping matching will improve, that Apple's agreement with the RIAA forced them to set a pretty high bar to what will match (since doing so gives you access to Apple's copy), but that as they roll the service out, they will tweak the matching process so more legitimate songs do get matched. Problem is that even if they do that, there's no guarantee they will just automatically re-scan everything that has been uploaded and replace it with a match.
I see you mention using a fix that involved creating a playlist and populating it with AAC versions. The information you've provided is a little limited, so bear with me if what I say here really isn't relevant to your situation but . . .
Getting iTunes to take a fresh look at a song isn't in any way tied to it being in a playlist as far as I know. The songs have to be removed from your library AND from iTunes match, AND a fresh copy of your database, one without those songs, needs to be uploaded.
So, if you didn't delete the originals from your library and the cloud, then run iTunes match so your database was updated in the cloud . . . and then with iTunes match off, re-import your songs into iTunes, create the new AAC versions, delete the old copies and then turn on iTunes match, then the chances are pretty good that during the matching process the most that might happen is that your new copies would get flagged as duplicates.
I know this from experience. I've been toying with getting vinyl rips to match and learned pretty quickly that if I simply replaced a file in iTunes with one I'd changed, one that Match had uploaded in the past, it didn't upload a new copy, just flew past it. Once I got the hang of it, I was able to get Match to really look at the new file. Sometimes it re-uploaded it, sometimes it matched, but main thing is it took a look.