man is your question timely! i have the exact same setup as you plus a walther v-drive.
i just replaced the transmission
cable after accidentally discovering the cover was pretty frayed. then i tried to adjust the cable. couldn't get the shaft to stop turning in neutral.
so i removed the cable from the transmission
link and just manually moved it forward and back, trying to find a 'null' spot. there was none.
i could feel a 'clunk' as i pushed the link into forward, and the shaft began spinning much faster. same when putting it in reverse. but where 'neutral' should be there was still a small amount of shaft rotation, maybe 50 rpm
i've decided to leave it alone.
but i do have an idea that i'd like to throw out there. could it be old or cold transmission fluid?? i'm thinking that when the fluid is thick it acts as a sort of clutch
. maybe when the fluid gets hot and thin the shaft stops turning in neutral. i'll have to check that next chance i get......