I have always taken the Isabella Spence one to be Isabella Spence McTavish who married Thomas, jnr.
Dates on headstones can be variable. Our common ancestor, Jean Spalding, is listed on the headstone as aged 86 or 87 in 1817 (from memory). She had her daughter, Jean, in 1793. Work it out - it's impossible. We decided that the mason had carved the figures wrongly or that daughter Mary, who had the stone erected in the 1830s, had worked out what her mother's age was at that time. Jean was definitely Jean's daughter!
I haven't got Isabella McTavish's death date to a hand at the moment - when did she die?
Gadget