Henry5
I know it is a little while since you posted but I thought I would reply as I think we are related. I am 99.99% certain that my grandfather is the William Kelly shown on the census reports i.e. your great grandfather's younger brother. Unfortunately I cannot help you with any information regarding William and Margaret (I am only just starting out on this genealogy business!). However my grandfather continued to live at Shandrum until his death in about 1973. His wife, Susan Fullerton, lived on in the family home with my uncle and his family until she died at the age of 101 in 2002.
My mother was one of nine although two of her siblings died in childhood. Four of my aunts and uncles still live either in Shandrum or within a few miles. My mother and two of her brothers moved to Birmingham in the 1950s.
Going back to the census details, I met the youngest child, my great-uncle George, when I was very young. By then he was living in Boston. I believe that one or two of your great grandfather's sisters also went to America (my mother, now passed on, told me this years ago but I didn't pay much attention then, and I have seen sailing records that may relate to them). I think another brother did the same.
I hope this is of some use to you.