Do Scotland have a mechanism to overturn a Westminster NO to independence?
This is a very interesting question and the answer seems to be yes.
Self determination is entrenched in international law and embedded at the individual level in the European Convention of Human Rights, which has direct effect in UK through the Human Rights Act and the Scotland Acts.
One path to overturn a Conservative rejection of a Section 30 Order then could be:
1. Achieve a nationalist majority at Holyrood next May.
2. Request a section 30 order again and if rejected,
3. Call a referendum on the question 'Should the Scottish people have the right of self determination?'. If the Scottish people vote that they should,
4. Commence legal proceedings against the UK Government under the Convention.
5. If the court case is won, the Court of Session (unlike the equivalent court in England) has the power to assume the powers of the Government and grant a Section 30 Order even if the Government itself opposes it.
6. A Scottish Referendum is held on Scottish Government terms.
The path is there is law. The only question is whether the people of Scotland want independence or not. Cameron realised this which is why he agreed to the referendum in 2014. The current UK Government is so inept they will probably fight to the bitter end making so many enemies on the way that independence is made much more likely.