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:
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.