I think we may have corresponded previously.I have recorded the Strathdon OPR for Penelope Stuart, daughter of James Stuart and Isobel Rannie, wife of my 2nd-gg-uncle James Grassick.In her lifetime, Penelope only used the maiden surname Allanach. Marriage and/or death records of her children, which took place before Penelope's death all give Allanach as her Penelope's maiden name. However, after her death, Penelope's children all used Stewart as mother's maiden name.Most interesting is that daughter Helen, who gave her mother's maiden name as Allanach on her 1st mariage in 1864 to William Dallas, gave mother's maiden name as Stewart on her Nov 1869 marriage to James Gordon; and this only 6 months after Penelope's May 1869 death.
Also, Isobel Rannie, wife of James Stuart/Stewart, only used the surname Alnach or Allanach on all of the census records 1841 thru 1871.However, on her 1873 death record, where the informant was her daughter Ann Thomson, who gave mother's name Isabel Stuart, widow of James Stuart, daughter of Arthur Rannie & Ann Gibbon.
So to my mind, it depends on which generation is giving the information for the records. Penelope & mother Isobel never officially used the surname Stuart or Stewart, only their descendants did.
I too unable to determine James Allanach/Stuart/Stewart's date of death or parents. Best I can figure is he died sometime between 1834 and 1840: 1834 being the birth of last known child William found on the 1851 Alford census at Archballoch (or is this Wm s/o Robert Allanach & Mary Hay???); and 1840, because he (James) is not on the 1841 census with Isobel, who is on the 1851 as widow.
I have much on the descendants of James Allanach/Stuart/Stewart and Isobel Rannie as have been in contact with descendants of several branches of the Allanachs.Would be delighted to exchange gedcom files if you are interested.