I have the same power plant a and martec folder, 12x6 I think, on my J28,
7500lbs bare. Forward works fine but the motor/prop vibrate a bit at certain rpms. Reverse is a suggestion, not a command. I get 5.5 knots at 2500rpms in forward. Reverse does hardly anything until it actually starts moving in reverse and the blades open up full. Then it moves fine backward.
It sounds like you props may not be opening properly. Any kind of crud will really effect the prop function. A few times I've noticed "bad behavour" with the prop and a quick dive and clean off straightens it out. Based this being the end of the season and may the problem wasn't noticed early prop crud is my guess.
Do you think early prop crud will cause the engine to die when placed into fwd?