Hello sonofthom,
I am not going to be very helpfull, but I have McKellars in my tree in the Dunoon- Kilmun area. They appear to be very numerous and the christian names re occur with great regularity so it is highly likely there is a common relationship. I have found McKellars buried at Lochgoilhead, and apparently my ancester Margaret McKellar nee Campbell, was buried there. Her husband was Peter b abt 1771 and they had children named Duncan, Catherine, Helen, Ann, Peter, John etc born from 1801 to 1820. This is later than your family but shows the continuity of family names.
If you can solve your problem you may be able to help me solve mine.
Wyanga