The cracks under the crossbar are certainly a problem. There were quite a few 18s with cracks like that between about 84 and 89 when we finally got the boat bullet proof again. There is a stainless steel reinforcement plate that should be (should have been added) there (inboard and outboard) beyond that, glass on the inside that would connect the deck to the sidewalls and going for and aft (12-14" total) would be good reinforcement. I'd say $300. is pretty good as the alternative is a failed bow and total loss of a hull.

The inboard bolt threads into a stainless nut that is mounted inside a casting... which is riveted inside the crossbar. You can replace the nut or whole inboard support casting if the bolt fails or you have to cut it to remove the crossbar. The replacement parts are shown in the Hobie Parts Catalog. The 18 is hard enough to take apart without age and corrosion making it worse. You may be able to tweak the hulls around a bit and ease up the tension on the bolt. Lift one bow and then the other a little while someone tries to turn the bolt. Try a hammer tapping on a socket wrench while you turn it. That is a bit like an impact wrench and can sometimes break a bolt loose.