Suggestions?

Yeah, if you can't figure out a way to access the inside, make some access. Cutting a section out of the side is actually very easy. And since you're going to strengthen the insides anyway, there should be no worries about weakening the structure. Matter of fact, gluing the section back in results in that area gaining strength, not losing it, if you do it with that consideration.

Strengthening a firewall that has a history of pulling out really won't be of much value from the outside. That is, unless the thing is made of liteply. Come to think of it, I haven't seen a single Chinese/Vietnamese mfg'd ARF that didn't have a liteply firewall..... so....... it is a good idea to laminate an additional layer of plywood. LHSs sell real plywood. You won't need thick ply. Real plywood is hard enough to do stand compression that bolt on motor mounts create, and 1/16" is enough.