I'd say in general children will be most comfortable in the "right" class for their age, in which case she ought to start P2 in the August when you move back.
The reason is that I think most children don't like to stand out as being noticeably older or younger than their classmates. It's different for the Jan/Feb birthdays where it's about equally common to start early or defer, but with a September birthday in Scotland, she would be much the oldest if she started P1 at almost 6. A boy in my son's first class did this, because his parents felt that we start school too young even in Scotland, and he always stood out as larger and a bit awkward in some ways with the rest of the class.
She would soon catch up on the work, especially if she has already done some letters and numbers at nursery, so I think the social side and the longer term effects would be the higher priority.
As to applying from abroad, I don't have any experience, although when we applied we had not yet moved to the area and were treated as a "placing request". We did not have the same rights to a place as we would have done if we lived in the catchment, and this could have been an issue if the class was already full - there is a maximum class size in Scotland which they will not normally exceed, even if you move into an area.
If you know which area you'll be moving to, then the education department should be able to give advice on how and when to apply, and whether particular schools are likely to be a problem.