The issue at discussion is, is Somaliland seceding or is it merely dissolving a union.The actual legal case for Somaliland's independence is not up for debate, beyond the AU fact-finding mission mentioned by Xildiid above, the Office of Chief Government Law Advisor of South Africa issued their official opinion on the issue of Somaliland's independence, clearly stating that Somaliland does qualify for independence as a sovereign state. This is the law advisor of the government of South Africa.
The issue of Somaliland's independence has more to do with international indifference than proper legal backing.
Though your referencing of Quebec is interesting, as this is absolutely worst case scenario for Somaliland, which is a referendum held across the whole territory of Somaliland (like Quebec) on the question of independence, and we know what that referendum will return.
The legal question would be, is Somaliland entitled to dissolve the union of the State of Somaliland and the State of Somalia, that resulted in the creation of the Somali Republic?
Somaliland's legal case rests on one fundamental issue that is "can the 1961 retroactive Act of Union be legally binding?". Provided this Act of Union is found to be void due to its retroactive effect then legally the State of Somaliland (and equally the State of Somalia) still exist as legal entities and there is in fact no need for Somaliland to dissolve a "union" because there was never a union. If however, this Act is binding then this means Somaliland has no legitimacy to dissolve it. Am of the opinion it was legally binding, the document Somaliland references to thwart and counter this, is the Vienna Convention on the Law of Treaties which is a very weak argument. The treaty entered into force in 1980, Somalia is not a party to it and additionally the treaty does not apply to any treaty that was signed before 1980. The treaty also states that a treaty which within its provisions stipulates that it is intended to act retroactively - is binding. The Act of Union makes it very clear that it intends to act retroactively.
The case Reference Re Secession of Quebec, [1998] 2 SCR 217 is concerned with the question can a territory that is part of a sovereign state unilaterally secede? - Such as Somaliland and its parent country Somalia. The court held that it would be illegal both in domestic law and international law for such unilateral succession. Thus, if the 1961 Act of Union is a legally binding document then similarly Somaliland is unilaterally declaring Independence and seceding - which is illegal. The same issue happened recently in Catalonia in Spain.
A referendum has nothing to do with law. A referendum is designed to provide an indication of the public's desired political settlement. For example the referendum that caused Brexit is not legally binding, is the enactment of the European Union (Notification of Withdrawal) Act 2017 of the Westminster parliament that triggers the process to exit the EU. Therefore, Somalia must first allow for the referendum to occur and then pass an act in parliament to allow Somaliland to be an independent country.
Both the report of African Union fact finding mission and the Office of Chief Government Law Advisor of South Africa are not binding legal documents and therefore cannot be used in a court of law. Additionally, both documents only state Somaliland 'qualifies for statehood'. They do not elaborate on how and why it legally qualifies for statehood.
Nobody disputes that Somaliland qualifies for statehood, all that is required for statehood according to the Montevideo Convention on the Rights and Duties of States is;
(1) a permanent population
(2) a defined territory
(3) a government and
(4) The capacity to enter into relations with other states.
Somaliland meets all four points, but so do Puntland, Jubbaland, South-western, Hirshabelle and Galmudug. That is if they wanted to be independent countries of course.
This is why Somaliland is considered a de facto state (existing in reality) as oppose to de jure (existing in law).
Qualification for statehood and sovereignty are two different principles. Whilst the former is concerned with the bench mark required to form a state and later is concerned with legality and territorial integrity.