If parity is valid, and the by way you describe what happened it should be, this should work to rebuild disk2:
-Assign the new disk2
-Important - After that leave the browser on that page, the "Main" page.
-Open an SSH session/use the console and type:
mdcmd set invalidslot 2 29
-Back on the GUI and without refreshing the page, just start the array, do not check the "parity is already valid" box, disk2 will start rebuilding, disk should mount immediately but if it's unmountable don't format, wait for the rebuild to finish and then run a filesystem check