The accent is not likely to be an issue at that age. The kids still studying for highers or advanced highers are generally the better behaved ones, it might have been a concern in lower secondary. It's really something that would be low on a list of concerns compared with her academic challenges.
If she turns 18 in Jan 2025 then she would have finished school in Scotland already and be moving to uni this autumn - however many children born in Jan/Feb deferred at start of primary so she will not be the only older one of that age if kept 'back' in S6. School year age groups run March to Feb, with a lot of overlap of children born from Xmas to Feb being in year below.
I don't know anything about the Canadian curriculum, so a lot will depend on her academic levels. She could technically do Highers in S6 but schools will usually want a certain level of achievement in the subject at Nat5/or GCSE coming from England - it sounds like she's not got any qualification yet? Students planning towards university in Scotland will typically do exams in S4 (Nat 5s) S5 Highers and S6 more Highers and/or Advanced Highers. So her peers would already have a lot under their belt. MFL is very much in decline in the UK sadly so take care choosing a school that can provide the necessary language options. They don't all even do French any more. She will likely be well ahead of her peers - is she bilingual?
School starts in about 3 weeks time. However Scottish schools actually move up a year in June and do a full month of their new courses so she is likely to have missed a fair bit of content already.
If you genuinely plan to move her I honestly don't know how you can make it work for S6 in 3 weeks time - schools are closed for summer. Someone already recommended the parent with job offer going ahead and other one staying with her to finish final year in Canada. Awful as being apart might be it honestly sounds like the best plan for the sake of less than a year.
Or a thought... there are more and more online schools here now - mostly for GCSE rather than Scottish curriculum - maybe there is an equivalent Canadian one you could use? If you can make the time zones work. Or some are asynchronous recorded lessons with live tutor support. And then just take her back to sit the exams?