Can the neutral for a spa circuit be tied to either the neutral on a return leg or ground?
The answer to both A &amp; B is NO.
The only place the ground and neutral can be tied together is in the main panel if under the NEC.
There is a 3rd way pull 4 wires.
Not wanting to pull a 4th wire would be a very shaky legal defense if someone bootlegs a ground and there is an injury.
If you only power a 240v load a neutral is not needed and that 3rd wire can be your ground but no tying neutral and ground together.