There is no time that the NEC allows the use of an EGC as the neutral on the load side of the service disconnect.
The NEC used to let you use the neutral to ground range and dryer frames. This is not allowed anymore except in existing installations.
Current carrying conductors must be insulated, the EGC is not insulated.
One reason for the separation of the grounded and grounding conductors is to prevent a difference of potential caused by voltage drop across the conductor.
It is great you want to learn the reasons for this but you should (in my opinion) not believe you can second guess issues that have long been worked out.
If it was not a safety issue the NEC would allow it.
Bob
