Worst case, do a COCOA install:
Download the FULL installer: Flash Player Plugin (Mac OS X)
Save it and quit Safari.
Double click the DMG to mount it
DO NOT RUN the installer!
Right click (Ctrl+click or 2 finger tap) and "Show Package Contents"
In Contents/Resources, right click the Adobe Flash Player.pkg and select "Open".
You'll see a warning. Click "Open" and run the installer.