I just looked up Absalom Boyd and Marion Spalding (b.1802). Marion died in 1828, when her daughter Joanna was one year, so Eliza (b. 1859) could not possibly have been her child. Also, since Joanna Boyd Elliott's youngest child was born in about 1871, it stands to reason that her birth couldn't have been a mere twelve years earlier.
I thought it was interesting that Joanna named her first daughter Marion - maybe as a way of remembering the mother she didn't remember? It's a sweet thought.
Do let me know if you have any more information on William D. Elliott's ancestry.