It is very strange the highly variable life of the SM manifold.
I took ours (20 years old) out last year, cleaned it, checked the metal thickness, and in every way it was like new. Other than a bit of green patina on the outside surface, it looked as though it had been manufactured yesterday. I gave it a new coat of black paint, and it is back in. Unless something changes dramatically, it should give us good service for many more years.
Both the main seacock and the manifold should be connected to the bonding system, although the presence or lack of of bonding would have no impact on _external_ corrosion of those parts.
Maintenance might have something to do with it. Allowing salt to sit against the manifold from a tiny leak at the hose joint would be a bad thing. Stop the leak, and be sure the solid salt is rinsed away. Such salt can create a conductive path from the stainless hose clamps to the copper manifold that would not be a good thing.
I have heard that some people will drop a "chorine tablet" (Calcium hypochlorite) into the seachest to discourage bio-fouling. This would also explain a short life for the copper manifold (and any other copper parts in the seawater piping!) and is (in my opinion) a bad idea.
Otherwise than those guesses I have no idea what happens to some of these manifolds... copper should have a very long, almost indefinite, life in seawater service.
Fort Lauderdale, FL