Hmmm. Well the answer to this is that the Brexit referendum also should have required a super majority, which is what I was arguing from the beginning before it was even declared. Major changes to the constitutional arrangements of a nation should require more than just a simple majority in my opinion. USA also seems to agree since they require two thirds of both houses before changing the constitution.
I understand, but legally, the Scottish parliament has been devolved certain powers that it is allowed to exercise. Those power may or may not include holding a referendum. They do already exclude making a decision for Scottish Independence but I don't know the wording or extent of that. If they don't include holding a referendum, it means the Scottish government would be technically overstepping their powers and spending public money in a way that they are not legally allowed to do. I'm tempted to say that actually, yes the Scottish parliament is in fact a committee, albeit a very large one with much more sweeping powers than a local council. It is not a sovereign parliament that can decide any law it wants (otherwise presumably we wouldn't be having the debate in the first place

)
To use an extreme example, this would be the same if the Scottish government today decided to set up an army and start purchasing fighter jets - they don't actually have the legal power to do that under UK law and they are still currently part of the UK. Once fully independent, they could then start doing so.
If they are overstepping their powers, such a referendum could be halted by a court order, which is different to being halted by the Westminster parliament. This court action could be brought by anybody, not just the UK gov't, so it's a relevant point to whether they would try to execute such a plan.
Now, whether this would be a politically good idea is a separate question and I tried to make that clear.
(This point also illustrates the question above about sovereignty - the EU cannot stop the UK holding any referendum it likes, but the UK can legally stop Scotland from doing so, in theory at least).