Agnes Stewart is presently identified as the daughter of Andrew Stewart, Master of Ochiltree, and the husband of Gilbert Lamont of Knockdow. Stewart of Ochiltree did have a daughter named Agnes, but it would appear that she never married.
The Scots Peerage. Volume VI
https://archive.org/details/scotspeeragefoun06pauluoft/page/516/mod...
Gilbert Lamont of Knockdow did marry a lady named Agnes Stewart, but Hector McKechnie claimed that she was the daughter of Stewart of Kelspoke, and that she died in circa 1636.
The Lamont Clan
https://archive.org/details/lamontclan12351900mcke/page/421/mode/1up
Gilbert Lamont of Knockdow's marriage to Agnes Stewart, sister of James Stewart of Kelspoke is confirmed by the charter that follows:
26 November 1601: Charter following on Contract of Marriage between John Lawmonth of Knockdow and Gilbert his son and apparent heir, on the one part, and James Stewart of Kelspok, and Agnes Stewart his sister on the other part, of date at Rothessay 14 March 1601, whereby the said John Lawmonth obliged himself to infeft the said Gilbert in the lands hereby granted, namely, the four merk lands of Inverkallen, two merk lands of Knockdow, four merk lands of Strongarok, and three merk land of Towart Easter; extending in whole to a thirteen merkland. lying in the Lordship of Kowall and Sheriffdom of Argyll: To be holden to the said Gilbert and his heirs of the granter and his heirs in fee and heritage; rendering therefor yearly one.[ ? ] of usual money of Scotland, in name of feuferme, on the ground of the said lands: Reserving to the said John Lawmonth the frank tenement or liferent of the said lands of Knokdow, Innerkallan and Towart, and to Marjory Campbell his spouse during her lifetime the frank tenement of the two merklands of Knokdow. Dated at Knockdow 26 November 1601. Witnesses, Gilbert Lamont of Coustoun, Duncan Lamont of Garochar, Archibald Lamont in Dunloscan, Thomas Stewart and Mathew Spence, notaries. The granter's Signature is wanting and the notary attests that he cannot write. "Duncan Lamont witnes" subscribes.
Lamont Papers: charter number 375 on p. 119
https://archive.org/details/scottishrecordso54scotuoft/page/119/mod...