And somewhere in that system you will find something abnormal with a neutral. If there was not something wrong with a neutral somewhere there would be a lot less than 25% current on the metal water pipe although I do remember the shielding of coax being added to this discussion at some point in time as well as the grounded conductor of a telephone system.
No that is with a perfect neutral. Remember that we are often talking about copper pipe that has a metallic connection to a cast iron main. The resistance of this path may very well be less than that of the grounded conductor. It is the fact that it is a remote path that increases the impedence and reduces the current on the water pipe system. Also, this is not common in newer areas as the bell and spigott connection with a rubber gasket on the cast iron mains breaks the path.
As far as the telephone and cable paths, they are of much higher impedence and have little current on them. In the case of the metal underground water piping system, you can lose the service neutral and not even know it as the pipe will provide the path for the neutral current.
If you lose the neutral and do not have a path via a metal underground water pipe, the current on the TV cable shield and or phone shield may result in a fire. As I have said before, I was involved in two fire investigations were there was not a metal water pipe ground and the service neutral was opened. In both cases the cable bonding wire between the cable shield and the electrical grounding system carried so much current that the bonding wire got hot enough to ignite the wood.
The amperage on the frame of the generator will be in parallel with the neutral of the system. Should someone make contact with the frame while standing on the ground they now become the third node of this parallel path. The amount of current flowing through this person will be solely dependent on the amount of resistance between where they are touching the frame and the grounding electrode system of the premises system being served. The amount of resistance will depend on such things as the weather conditions, the type and condition of foot ware if any, the type of grounding electrode system, ect??..
What voltage is available to drive this current?
If there was no danger then the NEC would not mandate the differences between a SDS and a non-SDS. Remember that the NEC is a minimum safety standard and to do anything less will constitute an unsafe installation.
The NEC is also a fit all document and there is no way it can address every possible condition. I see very little danger in having the grounded and grounding conductors in parallel for a small portable generator.