I would not like to have your "nightmare" but it is not an impossible situation.
For the nut/locknut I would use two spanners (you call them wrenches?) of the correct size and suitable levers after sizzling it all with a blow torch if you want to have the satisfaction that you were not defeated.
You can also do away with the idea of the stuffing box altogether as I have done with my own boat.
Fit a shaft seal
instead. Mine is a Volvo
. It does not last forever but then what does?
It needs to be changed every five years or so.
If you don't know what I'm talking about have a look at this
Cannot say for sure but the 'hole" on the coupling is probably filled with 2/3 split pins as I had on my Gib'Sea
364. There should be the exact same hole on the other side of the coupling. If this is the case you need a punch or a steel
rod the exact size of the diameter of the hole and a heavy hammer, to free your prop shaft, which must slide back for the stuffing box to come out and the seal to be fitted.
It is easier doing it than talking about it.