Everyone, thank you so much for all your replies! I am an idiot.... the Janet who married in 1955 is, indeed, Janet herself. I had too many death certs open at once and got Janet's and her mother's mixed up (Janet's spouse John McCallum registered her mother's death). The mystery of her marriage is solved! I'm still conflicted as to whether it was my Janet who emigrated in 1924, (
https://www.findmypast.co.uk/transcript?id=USC/1930/004952044/00683/0170) - Janet's aunt Margaret emigrated at around that time (according to some rather vague notes my late grandfather made in the 1940s) but it's equally possible that I've muddled up two people with the same name. I'll do some more digging on that, but honestly thank you all so much for taking the trouble to reply and sort me out!
Added as Carole's reply came in as I was typing - I had indeed been tying myself in knots looking for a daughter who doesn't exist. Thank you all so much for your help!
Also to add, in common with a lot of families the same names crop up time and again as I'm working through the Peddie side of the family - it's an object lesson in double checking the dates and surnames carefully! I feel a right fool, making such a rookie error....