// There is no "must" about it and any chaos (which in fact amounts simply to the UK leaving at 11pm tomorrow) would be of her making. //
The UK has already agreed to the extension in international law, so the chaos would amount to the UK's national law being contradictory to international law. I am not going to claim that one takes precedence over the other, but that the contradiction exists would be chaos enough.
Elsewhere I provided what I believe to be a refutation of the claim that the Statutory Instrument had to be passed before Theresa May could agree to an extension. In summary, the relevant passage in the EU (Withdrawal) Act 2018 (Section 20(4,5)) makes clear that the SI can only be laid to amend the definition of exit day to ensure that it aligns with the Treaties -- which is to say, two years after Article 50 Notification *unless* Article 50(3) applies, ie after reaching a unanimous agreement with the other member states to extend this period.
Now what certainly *is* true is that I haven't yet considered what would have happened had the Sec. of State for ExEU not laid the SI. Then, presumably, the definition of March 29th might still apply, so that the Section 1 of the 2018 Act, repealing the European Communities Act (1972) would still apply on that time. But this is now a hypothetical matter.