I even thought if I could maybe try to download 6.0.5 or even 6.0.4 again on to my Mac it might start working,
That would not help, sorry. You need GarageBand 10.0 to open GarageBand 2.0 iOS projects on your Mac.
This is not what their support page for iOS Garageband says "...will not work with Garageband for Mac version 6.0.1 or earlier...must us 6.0.2 or later."(http://support.apple.com/kb/HT4574)
Please send feedback to Apple Support, that the information in the document is incomplete, misleading, and needs updating urgently.
And consider to use the "Helpful" rating buttons at the bottom of the document. http://support.apple.com/kb/HT4574