So after some digging and ignoring family trees on Ancestry as they just don't seem to add up, I thik I have a timeline..
1499 or 1502 - Birth, location unsure
Marriage to Alexander Scott, as yet unable to source a date and place but assume before 1519
1519 - daughter, Elizabeth Scott born
1520 - some but not all sources have a son being born, Alexander?
1539 - marriage to George Henderson, 2nd Laird of Fordell....quite a few sources mention this date, heraldic books, etc.
1541 - daughter Janet Henderson born
1547 - daughter Marion Henderson born
1566 - death in Glemsford, Sussex on 20th April
So the marriage to George in 1539 rules out the information that says MQS was at the castle for the wedding of Marion and George as Mary wasn't born then. I do however wonder if it was Mary of Guise (Queen to James V and mother to MQS) who was actually at the castle for the wedding and she was in fact her lady in waiting? Obviously this is all speculation. After all being lady in waiting to MQS has more kudos than being lady in waiting to her mother. This though would make much more sense.