My understanding of the purpose of the interrupter is that because of the
design of the clutch dogs on the sliding clutch ( back cut 2 or 3 degrees or
somesuch), it is necessary to release the torque applied to the clutch to
make it easier to disengage same. To prove the point, disconnect one of the
wires on the interrupter microswitch and notice how much more difficult it
is to disengage gears while the engine is running. Also notice that there is
a tendency to overshift into the opposite gear.

Older Mercruisers didn't have the back cut clutch dogs and had a tendency to
fall out of gear at idle speed as the dogs became rounded from wear.

Earlier I made mention of a shift shaft seal. What I was referring to would
be more correctly called a bell housing bushing seal.

The original poster didn't say if he was having difficulty pulling the
gearshifter into neutral. That would be key to knowing if there was an
interrupter problem.
Jim