Eugene kindly referred me to fabpedigree website but As I have found this source to be less than accurate in the past, I researched some more.
What I found seemed far more likely. First, as per previous exchange, Joanna appears to be the same person as Jonet and Janet... so yes, I would argue that you might care to make that change.
I then found that Joanna was married to John Gallda MacDougall. My research is thus -
Joanna (or Johanna) Isaac, granddaughter of Robert I the Bruce, was born in or about 1337, the daughter of Princess Matilda (Maud) Bruce and Sir Thomas Isaac. She lived in the time of the Bruce's son, King David II.
It is almost certain that King David II was responsible for Johanna's marriage to his favoured knight, John Gallda MacDougall. The union itself is datable to at least 1362 and resulted in the king regarding John Gallda as ‘nephew’. In theory the marriage meant that any children of the couple potentially possessed a claim in the royal succession, and this would have been seen as a threat by the leading claimant, and eventual successor to David II, his nephew Robert Stewart. Royal acts of the time reveal that David II bestowed much favour upon John Gallda and Joanna.
After David II died in 1371 to be succeeded by Robert II, the MacDougall's fell completely out of favour.
According to Gesta Annalia II, John Gallda and Joanna indeed had a son, or sons, but they certainly never succeeded their father, and evidently died young. What is certain, however, is that the couple had two daughters: Isabel and Janet. At an uncertain date, Isabel married John Stewart of Innermeath, whilst Janet married John Stewart's brother, Robert Stewart of Durisdeer. The brothers were members of the Stewarts of Innermeath, a cadet branch of the Stewart family.
According to Gesta Annalia II, after John Gallda's death in or about 1377, Joanna married Malcolm Fleming of Biggar, a union which appears to be corroborated by a papal dispensation granted in 1377. This marriage and the marriages of John Gallda's daughters to the Stewart brothers saw a slight neutralisation of the relationship with the king.
