We had same meltdown of Vetus plastic muffler last year. Cause was worn drive adapter on cam shaft that turns in the slot of the sea water pump (Perkins M50). Fixed similar to you with pieces of neoprene, 4200, a piece of Katherine's plastic cutting board and lashing with a Spanish windlass. Had spare water pump.
Yes, surprising how quickly the Vetus plastic melts with no water flow - definitely an argument to have an exhaust temp sensor as waiting for the engine overheat alarm was too long for both of us.
Craig Briggs, SN#68 Sangaris, underway south of Coinjock, VA