Hi,
I'm a Tayvallich amateur sleuth! Well, I'm an amateur family historian, who once lived in Tayvallich, but now live in Canada. I love solving family tree mysteries and I've done a few of my own, but sometimes it takes many years, be warned, to gather all the clues. My family is mostly Argyll based too.
I was looking on ancestry.ca and I found a birth for a Henry Thomson on 17 April, 1840 in North Knapdale, Argyll (where Tayvallich is) to Harriot McIntyre and Henry (or Hendry) Thomson. If Harriet and Henry were split by 1840, maybe the local minister knew something that the registrar didn't ... that the new baby was not Henry Sr.'s child at all??? Just a thought - there are plenty such occurrences and cover ups in my own family tree.
As for Henry Jr. being a grandson of Bell Grahame, I suppose it is possible in several ways (1) Bell was a Mrs/Miss McIntyre before she married Angus Grahame, (2) a son of Bell's was the actual father of young Henry or (3) Harriet was born a Grahame but had an early marriage to a McIntyre or (4) she simply thought of him as a grandson, but I doubt that one.
Where did Henry Jr. go after he grew up? I see him (I believe) on the 1901 census living with a wife and daughter, both name Flora, in Kilcgreggan & Cove, Dumbartonshire. He gives his birth place as Tayvallich.
Oh and here's something very interesting I just found, that hints at my first idea being more probable! On a report (I will provide the link) of local affairs it states " I met a blind man of the name of Angus Graham, age about 56, who informed me that he gets ..., pays no rent, has one daughter in service in Greenock, a wife alive and with him ... A boy ten years of age lives with them, a grand son of his wife, deserted by his parents."
http://books.google.ca/books?id=8HZbAAAAQAAJ&pg=RA1-PA58&lpg=RA1-PA58&dq=angus+and+bell+graham+north+knapdale&source=bl&ots=32AHDXbl63&sig=VGkOvuKi801RD1lvxwjl_ThvgZc&hl=en&sa=X&ei=-JXmUr6bOoO42wWN4IGgDw&ved=0CGYQ6AEwCQ#v=onepage&q=angus%20and%20bell%20graham%20north%20knapdale&f=falseHope that helps some. Let me know what else you've got. I would love to help you unravel this.
Mary