Noooo! Don't cut out the rib.

You can tunnel in from the bottom, just behind the root rib. Cut out about 1/4" x 1/2" of sheeting right under the blind nut, and use a screwdriver or something to dig out the foam. Then you can get to the blind nut and replace it.

To fix it just cover the hole with ultracote. No need to replace the sheeting, there is absolutely no stress in that location because the wing tube is supporting everything a foot farther out.

TF