I am not understanding what you are saying in the underlined part above. To keep it simple let's use a residential single phase service with two hots and one neutral. Where are the multiple paths at in one of those?
Around here all services are in metallic raceways. The conduit between the meter can and the service disconnect will be in parallel with the grounded conductor.
If you have TV cable, the shield will be in parallel with the grounded conductor, via your neighbors bonding connection. I have seen two fires that have resulted from this required parallel path when the service neutral was lost at the pole and the neutral current was running on the TV cable bonding conductor.
In areas with common metal underground water piping systems, the water piping is in parallel with the grounded conductor, again via your neighbors bonding connections. It is not uncommon to see 25% or more of the grounded conductor current flowing on the water pipe when you have a common metal underground water piping system. This happens even when there are no connection problems with the neutral. If there are problems with the neutral, more current will be on the water pipe...maybe even all of it. This NEC required parallel path creates a serve hazard for anyone cutting the water pipe between the GEC connection and the water pipe street connection if there is an open neutral. It is not a big hazard to the plumber if the neutral has no connection problems.
If you have an underground phone line, the shield will be bonded at your house and at other house creating another parallel path for grounded conductor current.
These are all code required parallel paths for the service grounded conductor current. Based on these required paths, I see little danger in a parallel path from having the neutral of a stand alone generator bonded at two locations. There is no question that there would be a code violation, I just don't see the danger.