The flaw in your logic is that the 5V potential difference between neutral and ground that you have assumed is actually no longer present once the neutral & ground are shorted.
Perhaps I should have taken an example:
A 25 mm2 (3 AWG) 3-wire cable going to a panel, carrying 100A. The voltage drop is 10V. Then I connect a 2.5 mm2 (13 AWG) wire between neutral and ground at the load side. If we assume that this wire is very short, the resistance can be neglected.
If the ground and neutral conductors has the same area, the current will then be shared evenly between them, right? The current going through the 2.5 mm2 wire is now 50A, right?
But, indeed there is no longer a potential difference.
