Been a few months since I last visited this forum, not much boating at this time of year here in England lol.
I wonder if - While the engine(s) under load the marine manifold gets warm/hot, perhaps not even excessively hot, maybe no hot patches. But then if you come off the throttle suddenly the fuel injected engine goes into over-run mode like a car engine under the same conditions would. There is a difference between over-run with the engine still injecting fuel so still firing and heating the exhaust and an engine that goes into over-run mode and stops injecting fuel thus suddenly stopping heating the exhaust. I don't know but might guess that the marine version isn't supposed to do over-run mode to prevent thermal shock to the marine manifolds. Picking at straws, I don't know if you suddenly come off the throttle or decrease throttle very slowly when the fault occurs.
I think at this point if you're going to persevere (and I don't see what choice you have other than fitting a different engine that uses a different marine manifold) I'd be thinking about fitting multiple temperature sensors to the marine manifold, at least say 5 temp sensors spaced across the length of it. Such temporary setup doesn't need to be very expensive, can buy thermistors for around £1 each and thermal glue them to the manifold, wire them all to the battery via a resistor, wire them all to earth, wire a separate multi-meter to the sensor side of the resistor that connects to battery voltage, then you have 5 separate temp readings. You can buy a cheap multimeter for £10 but you'd need 5. You'd get an idea of multimeter voltage readings versus actual temps by first testing with them wired up to battery voltage in a pan of water as you heat it while monitoring the temperature using any type of thermometer. This would surely show what was happening regards temperature of the manifold.
Shipping from US is not too cheap
