When you download Minecraft for Mac, you download an application file which you then place in your Applications folder. But that's just a wrapper for the "real" Minecraft binary files (a set of JAR files), which gets downloaded into your Application Support folder.
So when you delete and redownload the Minecraft application, you haven't done anything about the actual binaries already downloaded and being used. If these are corrupted (as they appear to be in your situation), they won't be fixed by redownloading the Minecraft application.
While "Force update" should do something about these files, you can do something about it yourself by manually deleting the downloaded JAR files.
To do this, you'll need to navigate to the path described in the error. In Finder, go to the Go menu and select Go to Folder... Type in the following:
~/Library/Application Support/minecraft
and click the Go button. Finder will open a new folder with a few subfolders, including one labeled bin. Throw the bin folder in the Trash and empty it.
Now go back into your Applications folder and open Minecraft. Minecraft will then redownload its binaries and the problem should be resolved.
Most fullscreen programs on Mac OS 10.8 will use the four-finger swipe to switch between it and the desktop, but Minecraft doesn't obey this protocol. It appears to use a borderless window the size of the desktop instead of creating and using a separate fullscreen space. It also appears to set itself to be always on top, so Minecraft won't relinquish focus when you use ⌘+tab.
The only way I've found to quickly switch is, as you have, to turn off fullscreen. Fortunately, there is a faster way to do this: F11 is bound to the "toggle fullscreen" function. If you're using a laptop then F11 probably defaults to adjusting your volume or similar, and you'll need to use fn+F11 instead. The fn key is usually located in the bottom-left, under shift.
Best Answer
The answer is pretty simple, you have black screen probably because Minecraft got freezed or stopped working, all you have to do, is click three buttons; command (the one right next to space), alt, and escape, then you should get a little window which says "Force Quit Applications", you should see all programs that are running, click Minecraft, then Force Quit, confirm that, and you're done! It worked for me.