"It is not in question whether Holyrood has the power to hold referenda. This has already been established thanks to the Referendums (Scotland) Act."
I know that and I definitely didn't ask if the Scottish Government and Holyrood has the power to hold a referendum on DEVOLVED issues, i.e., agriculture, healthcare, education, etc.
"What is in question is whether the SG can hold referenda on reserved matters, and whether there is a material difference between holding an advisory versus consultory referendum."
This is what I posed the challenge regarding. Neither Holyrood nor the Scottish Government can hold a referendum on matters they have ZERO power over.
There isn't a constitutional law expert who actually thinks the SNP will win their case in the UK Supreme Court.
"I suspect that the reason Westminster appears to have issues with this is nothing to do with competence, and rather down to the fact that Westminster, the Tory party specifically, have already set a precedent for making advisory referenda absolute and binding."
Your own speculation is irrelevant and certainly doesn't stand up by any means. The legislation is clear - advisory referendums are NOT advisory.
But the Doctrine of Parliamentary Sovereignty at Westminster still holds, so Westminster retains the power to implement it or not. That's why we had a democratic vote by each of our representatives in Westminster regarding this issue. It passed and so the referendum result was implemented.
Had MPs voted against it by a majority, the 2016 referendum would not have been implemented.
Your assertion that "Westminster, the Tory party specifically, have already set a precedent for making advisory referenda absolute and binding" simply does not stand. To argue otherwise is to use circular reasoning.
"What the Supreme Court ruled on the SG's involvement in Brexit negotiations is neither here nor there, as this has no relevance to the matter of how one Sovereign component part of what is claimed to be a voluntary partnership extricates itself from an incorporating union."
Actually, it IS relevant since the Supreme Court ruling demonstrated the devolved administrations were acting ultra vires. And that they are NOT sovereign states which is obvious.
Under international law, England, Scotland, Wales and Northern Ireland are NOT sovereign states. They are constituent parts.
The United Kingdom of Great Britain and Northern Ireland IS the sovereign state which was recognised by the EU only conducting negotiations with the UK Government.
Regardless, the SNP are clearly NOT confident they'll prevail against the UK Government in the Supreme Court.
I refer you to international law as it stands in regards to the issue self-determination as well.
"The Scots people themselves are sovereign, and as such have a right to dictate their own constitutional future."
Subject to international law itself as Catalan separatists found out.
"If Scots can not leave a voluntary union simply by a majority of Scots requesting to do so, then whatever the nature of mechanism preventing that is, it is self-evidently either fundamentally undemocratic and repressive, or the voluntary union is nothing of the sort."
Disingenuous argument as this is not reflected in reality. If that were the case, the 2014 referendum wouldn't have happened.
Since the UK Parliament is supreme under our constitution and always has been, it is NOT undemocratic for the UK Parliament and/or Government to say "now is not the time" or "no" to IndyRef2 demands.
The SNP has no right in domestic or international law to IndyRef2 simply because they demand one.
"The insistence that Scots can only be permitted to declare a wish to leave the union if Westminster consents to this is no more ridiculous than Germany, France, Italy, or any other EU member having ultimate authority over the UK's right to decide whether or not to remain part of the EU."
See above.
The comparison with the EU doesn't hold water as its not a country by any means.
Additionally, France explicitly forbids secession in Article 1 of its constitution in relation to French territory:
"France shall be an indivisible, secular, democratic and social Republic."
Germany's constitutional court has ruled in 2017 that it is a violation of the German constitution to allow secession:
"In the Federal Republic of Germany, which is a nation-state based on the constituent power of the German people, states are not ‘masters of the constitution’.
Therefore there is no room under the constitution for individual states to attempt to secede. This violates the constitutional order.”
Likewise, the Spanish constitution states:
"The Constitution is based on the indissoluble unity of the Spanish Nation, the common and indivisible homeland of all Spaniards...."
The United States of America is perhaps the best known example of a state which expressly forbids secession in its constitution:
"I pledge allegiance to the Flag of the United States and to the Republic for which it stands, one nation, indivisible, with liberty and justice for all."
And, in Canada, the Canadian Supreme Court ruled:
"It is clear that international law does not specifically grant component parts of sovereign states the legal right to secede unilaterally from their "parent" state."
Literally, Canada can simply say "no" to Quebec and this is perfectly legal in international law.
As you can see, it is in full accordance with international law that the Sovereign national power CAN forbid secession.
The United Kingdom Government and Parliament can say no to the SNP's secessionist demands as often as it likes. This is in full accordance with the international law as agreed globally.
That creates a real dilemma for the SNP who've already noted the EU's response in regards to Spain and Catalonia. Much to the chagrin of Scottish Nationalist separatists.
Whether we like it or not, the UK has agreed to an independence referendum for Northern Ireland, Gibraltar, Falkland Islands and Scotland in the last few decades. With the sole exception of Northern Ireland, the UK Government does not have to consent to further independence referendums for these parts of its territory.
International law decrees that demands for secession must be balanced against the right of sovereign states to territorial integrity.
Once again, the European Union is NOT a sovereign state and, therefore, territorial integrity does not apply to it. It does to each individual member state, but crucially each member state cannot dictate to the others in international law as it stands.
Your attempted comparison falls down.
"A comprehensive pro-Independence outcome in a GE would not, of course, bring about Independence in and of itself, but this isn't and has never been a matter where it's a case of 'ultimate goal, right now, or nothing at all'. It's another step towards highlighting the fundamental absurdity of the Act of Union, and the repressive nature of a system hell-bent on maintaining it regardless of the fact one member increasingly wishes to leave."
Except it really wouldn't highlight "the fundamental absurdity of the Act of Union, and the repressive nature of a system".
See the point above about various European as well as non-European states forbidding secession in their constitutions and court rulings. Its fully in compliance with international law whether you like it or not.
Finally, there is no evidence that Scotland "increasingly wishes to leave." If that was so, the SNP would have taken a lot more of the UK GE vote share than they did. It was significantly lower than their highest ever.
The last two polls had 59% and 53% opposed to IndyRef2. Further questioning in these polls reveals the people in Scotland are literally divided over the issue of an appropriate time frame for holding IndyRef2.
This does not correspond to an increased desire for independence.