Silicone caulk and 5200 are banned from my tool bags. Bought 4 rolls of butyl from Maine Sail to rebed all deck fittings. I used sikaflex on my through hulls, rope clutches and my mast step but everything else is buytl. It really is an amazing product.
Never really been a fan of buytl, I sell it, and some of my customers love it, but for me its not "stay put" enough .. it can be moved by flexing, it gets very soft in hot weather. I am personally a fan of 3m 4000uv for any deck work. It is 4200 (non perm) but with high UV additives so it will last longer and SCS2000 (GE product) for window type seals.
Beyond that.. I find this topic very interesting because 1 I am not sure those cracks can be associated with rust.. I think its just a failing chainplate - SO.. yes great idea to check them. But it also does show how much more abuse a saltwater environment puts on a boat. Here in S. Ontario we had more boats built in the 70-80s than any other area (save maybe CA) and most of those boats are still sailing with few if any critical failures, on original rigging and fittings.. yes we suffer the same core deck issues with water.. but the hidden damage from rust is almost no issue at all up here.
So when buying.. make sure you know the history of your boat.. where its been will give you a great idea of how diligent you may need to be. Glad everyone was ok.. hate sailing horror stories.
James