I'm such an idiot. The problem actually has nothing to do with Excel 2013 vs 2007. The computers that are running Excel 2007 have no user login as they are a shared resource, and the name given to the windows account has a "." in it! That was throwing off the InStr calculation in this step:

FileSaveType = Right(FileSaveName, Len(FileSaveName) - InStr(FileSaveName, "."))
Once I recognized that, simply changing the function from InStr to InStrRev fixed the problem!

I appreciate all your input though. Thanks!