Correct me if I'm wrong, NJ, but presumably a Minister of the Crown may only "lay by regulations" under terms that are explicitly laid out for them by Parliament? In which case the Minister *only* had permission to amend the definition exit day because it had changed in international law. The change in international law didn't have the effect of changing the UK's legislation automatically -- it can't do, of course -- but it created a contradiction between domestic and international law, and one that could only be resolved if either the EU revoked their legal assent to change exit day or, as has happened, if Parliament approved an SI to amend exit day.
So I still feel that, when you say "Mrs May was constitutionally out of order," this isn't true to the letter of the law. The permission to amend exit day was only given if it had already been amended in the treaties, and in no other instances. Now I may have misunderstood this but I did check this interpretation with a friend who understands legal stuff and this is essentially their argument rather than mine.
Where I *do* agree, though, is that this has clearly undermined the spirit of the 2018 Act, which was clearly meaning to fix 29th March as exit day. This was discussed on AB at the time, and it was clear from what meant Brexit supporters were saying, and clear, too, from my own frustrations at the fact that exit day had been so deliberately emphasised as tomorrow in UK law. It turns out that the amendment didn't even come close to having the effect intended, and makes rather a mockery of its even being introduced.