All very intriguing.
It is certainly true that Isabel's previous married name, if any, would not have appeared in the baptism records of any of her children by a second marriage.
I can find one child, Catherine, baptised 1775 to Donald McNeil and Isabel McGraffan, and seven, baptised between 1773 and 1796, to Donald MacNeil and Isabel Crawford.
Have you looked at all the original baptisms to see if they record Donald's occupation and/or the couple's residence and/or the names of any witnesses?