I'm a bit confused. Looking at the VD 72C manual both with and without reduction gear
I see that the splined output shaft is a solid piece that is internal to the transmission
. (Thus will have oil on the splines)
The coupling slides onto the output shaft and through the oil seal
. That is to say the oil seal rides on the coupling shaft.
The coupling nut is a flang type nut with a flat surface on one side that then acts as a seal against the main shaft.
If the sealing surfaces on the main shaft or the coupling nut are not flat or are scratched you can expect an oil leak
there. The manual does not specify using any locktight but does specify the torque.
Some part numbers for this nut are 10-00-149-034 or 4775L
Here is an online reference: ISSUU - Drivetrain by Mermaid Marine Products
I suspect that having a correct output shaft nut (coupling nut) will solve this problem.