RE: using expanding foam in built up wings
From what I understand about the product you are considering, I think it's likely it would expand so much, that it would destroy the wing; or, it would certainly distort it. You call them bulkheads but I assume you are talking about the wing ribs. Do I understand you to be saying 75% of them have been chewed on. The purpose of the ribs is to hold the structural parts of the wing in position and maintain the shape of the airfoil. I have no idea of the depth of the damage done. The mice probably used existing lightening holes in the ribs to gain access to the areas of the wings they damaged. How much they made these holes larger and how much this has weakend the structure is anyones guess.
My thoughts on making repairs would be the removal of the covering in the damaged areas to gain access and patching the damage. This won't be easy as you may not know what the damage ribs looked like before they were turned into mouse terds.
Sorry, but, I don't see a simple fix here.