How now brown cow? Floor repair project gets larger

Look's like your leak must have started at the top and ran down the corner. These things are built in sections. You could try opening the wall. You'll probably find more sandwich construction. You can replace the bad spots with a lot of ripped 2 bys and foam. Once your read to fasten the corner back in you can clamp it with ratchet straps around the outside. You'll need a place to work inside.