Round and round we go.

Can't really add anything to my original thoughts.
I've checked the census records again and the material is as quoted.

William (35-9) would seem to be the son.
IF he was a fisherman/mariner AND away on census night it would not be unusual to see his name added to the end of the schedule.

Although the ages of Catherine & Margaret are very clear on the return (5) either could be an Enumerator's error for 3.

Again, IF William was a seaman there would be nothing unusual in a child being born in England.