Debby,I agree with your theory about this Mary Orser.Here is what I have just found in an article that I have from the Whig-Standard newspaper dated July 29, 1976 re an Orser Reunion in Sydenham, ON: ..."Webster Walker displayed two cane bottom chairs made by local Indians, passed on to him by his mother, the late Mrs. Charles Walker, the former DELLA ORSER (my caps). The chairs were wedding gifts to her aunt, MARY ORSER, on the occasion of MARY ORSER's marriage to JAMES DOUGHERTY of Elginburg in March of 1851."
I have long had a post-it note with a big question mark on this article. Yes,I have had this Mary Orser,married to James Doughterty, as daughter to Samuel and Elizabeth and wondered how Della could be her niece. I think it now proves your theory about who this Mary Orser truly is, the daughter of Isaac and Miranda.Well done!