Well to start with, a search on Scotlands People shows no births for Ellen Higgins in Leith between 1897 and 1899.
So I would agree with findings in your previous posts, that the birth registered in Chorlton district 1898 is most likely. That covers Didsbury which is the birthplace you have on the 1901 census. I've no idea why your nan has given Leith as her birthplace, but I would guess that she has used the surname Cameron to avoid awkward questions about her parentage on her own wedding day.
My guess would be that David Higgins and Matilda separate (or David dies) and Matilda becomes Alexander Higgins's partner around 1902-1904. Then she becomes Wiliam Cameron's partner around 1912.
What I would do next is get some credits for Scotland's People, and find out about the birth of Ellen's sister Agnes Higgins. It looks very much as if she is not Matilda Kelso's daughter, as she was born in Scotland when Matilda is in Stockport. If she is Alexander Higgins' daughter by a previous marriage it will give you a bit more info, and you might be able to work out from that if David and Alexander were in fact brothers.
I did a search on Scotlands People and found an Agnes Higgins birth registered at Paisley, Renfrewshire in 1902. Would probably be worth checking out both the statutory and the Catholic registers, as it looks as if your family were RC.
Hope that helps

Barbara