I think it could be argued that the proposals are not affecting the law in the UK.
Scots law already varies significantly in matters of personal and family life including greater autonomy for 16-18 year olds. One could argue that ccWestminster recently interfered with Scots law by making Gretna Green marriages illegal for under 18 year olds domiciled in England and Wales.
A marriage conducted between 2 under 18 year olds who are domiciled in Scotland will be recognised as valid in England and Wales, even although they couldn't get married in England and Wales. An English under 18 could acquire Scottish domicile and be validly married in Scotland.
I think it could be argued the same way. The changes will apply to an under 18 domiciled and applying for a GRC in Scotland but an under 18 domiciled in England and Wales could not get a "Gretna Green" GRC.
So far as altering birth certificates, the legislation applying generally to this is a Scottish Act, so within the competence of the Scottish Government to amend.