Hi
Your surmise that William Stewart died bet. 1841-51 is correct as the 1851 census shows Grace Stewart to be a widow, aged 70, living in Glenalbert with unmarried sons William, a labourer aged 35, and Alexander aged 45, no occupation stated. All born Little Dunkeld.
Family Search shows a son John b. 17 Feb. 1805 Little Dunkeld to William Stewart & Grizel McIntosh.
A "marriage record" for William & Grace, if it exists, would at that date just be a record of proclamation of Banns. They do not usually show the parents of bride or groom. I would guess from the names of the sons that his father was John, and hers Alexander... but the traditional naming pattern does not always apply, so that is just a guess.
There are no OPR death records for Little Dunkeld, but burial records for the churchyard are held at Perth Crematorium (see Perth & Kinross council Bereavement Services webpage for details), and the book "Monumental inscriptions (pre-1855) in North Perthshire" contains inscriptions.
Isabel