In Kheldar's list of options for Scotland, I miss one:
Scotland could invite a foreign power to invade and liberate them. For example Russia.
I know it sounds weird (and the Russia example is even more weird), but technically: If a legitimate Scotish Parliament with a, say, 75 per cent majority asks for this (and it might base its decision even on an additional referendum) - could it be deemed illegitimate?1
This may sound like a joke (and actually it started as one), but interestingly the idea of foreign intervention seems not to be totally beyond (theoretical) consideration. A commenter said: "Indeed, the UN recognizes the right of people to govern themselves, Parliament has enough recognition to be considered to represent the people of Scotland, and we have witnessed recent 'liberations'."
An example for Russia having been welcomed to liberate a nation is Bulgaria (1877-1878, Russian war against the Ottoman Empire with Russia invading). Well, okay, one might dispute how welcome that war really was at the time (the part of the population that did not welcome it has been largely ignored by Bulgarian national historiography since), but indeed there were groups in what is today Bulgaria who actively faught for foreign powers to intervene so that the Ottoman Empire might retreat. Which indeed happened.
Can Scotland learn from Bulgaria?
Here is a twist towards reality: We do not believe that Scotland would really want to have a foreign power invading. However, only speaking out the invitation would already have an effect, namely on the field of public opinion. The UK could not ignore this clear vote to leave anymore. Moreover to be safe, Scotland could invite a distinctly small foreign power which does not really present a threat, perhaps San Marino or whatever. The power of the act would lie in its symbolism.2
1 Of course, it is hard to imagine that Scotland would call Russia for help. Perhaps we should consider other powers. Uh... Norway? At least they have some positive track record with conquering these islands...
2 Another twist would be not to invite a foreign power, but to declare war to it. And then immediately surrender. - I admit that this is not my idea. I heared it somewhere in late night comedy, although probably not applied to actually Scotland.