Hi Jean
I think your point is well taken that William might have emigrated. I also agree that Wm Wallace is likely not the right one. The 1900, 1910, 1920 census in US says he arrived mid 1880s in the US.
I have had a look in the IOM censuses for 1851-1881 to see if there are any William linked with parent John Watterson in the early census record that I can follow forward. I haven't got that all done yet - but there certainly are some candidates. By 1881 there are several Williams on fishing boats and so not linked to family records. I'm going to look further along this line though.

Really appreciate all the input.

Bev