Massive problems after changing a .plist file...

Help! After messing about with a .plist file, most applications fail to open.

Starting at the beginning, basically I wanted to get iBooks Author running under 10.6.8 so yesterday, I followed the following step in a guide:

From the Mac OS X desktop, hit Command+Shift+G and enter /System/Library/CoreServices/
Locate SystemVersion.plist and make a backup copy of it to the desktop
Launch the Terminal and type the following:

sudo nano /System/Library/CoreServices/SystemVersion.plist
Locate the keys ProductUserVisibleVersion and ProductVersion and change their strings from “10.6.8″ to “10.7.2″
Hit Control+O to save the file

It worked like a charm and I managed to install the app. I backtracked and changed the strings back to "10.6.8" and thought nothing of it.

Today, my system tells me that it wants to update iTunes and Safari, which I agree to. However, afterwards both crash and fail to open. I check my system version in "About this Mac" and it shows 10.7.2.

I go back into the .plist file and find that my changes haven't been made, so I go back through them and make sure I save. Now 10.6.8 shows up in "About This Mac".

Now iTunes and Safari won't open which I figure is just a case of deleting the Lion versions and downloading older ones, but the problem is that now a lot of other applications such as iCal and even 3rd party apps like Spotify fail to open. I get the following error for both:

"Spotify cannot be opened because of a problem.

Check with the developer to make sure Spotify works with this version of Mac OSX. You may need to reinstall the application. Be sure to install any available updates for the application and OSX"

Any suggestions?

 

Well the icing on the cake was that the app failed to work properly so I just deleted it in the end

I think something got changed when I updated Safari and iTunes...not sure what though

edit: should note that nothing else got updated, the apps which are now failing to work were the same ones I had running under SL just last night.

 

I think I'm just going to have to backup then do a fresh reinstall of SL. I don't know enough about the internals of the Mac to fix the issue myself.

Also the App Store also fails to open now so I can't download Lion