Re: Keel Bolts
If the bolts had deteriorated it would certainly have been visible when you removed the plywood, which was likely at least damp. If they looked good where they were buried in the plywood they will be good lower as well. You are over-thinking this. Adding extra bolts will not add much anyway as they will not have as solid of an attachment as the originals nor hold as much load as the originals.
Living aboard in Victoria Harbour