Okay, have not fully read the thread but let me say this.
No entity in the world qualified as a nation-state before 1700 approx, meaning Mongolia cannot claim nations it has conquered and even Israel's claim to the ME is the untenable.
For post-colonial nations, the story is markedly different. Since barely any nation in Africa qualified as a nation-state before colonisation, whether there was ever a united Somalia is irrelevant as even if there was, it would not be a state by the modern definition.
That is why the AU and European colonial powers, have decided that colonial borders should generally be the markers of modern African nation-states. Somalia is an anomaly, as both Somaliland and Italian Somaliland were almost entirely carbon copy nations with the same religion, ethnicity, language, culture. Furthermore, both colonies were together for approximately 10 years (1940-1950) and were split up with the intention of reuniting them in 1960.
Now, one would ask why Djibouti is not considered. This is primarily due to two reasons, Djibouti is a mixed country of Somali, Afar and Arabs, with a distinct enough culture. Furthermore, Djibouti, compared to the Somali populations in Ethiopia, Kenya, Italian Somaliland and Somaliland was relatively isolated. All the aforementioned colonies were together at one point. This in itself is a weak case, therefore, Djibouti was given a choice to join Somalia and the French intervened to make sure it never happened. That closes that chapter.
Another important thing to state is that international governance is not fair and never has been. It was and still is in the best interest of the IC, African Union and whoever else, to have Somalia and Somaliland join. The interest of Somalis never matters, that is why Ogaden is with Ethiopia despite being a weak argument.
As I said in another thread, the prevailing international opinion is that Somalia and Somaliland will negotiate after Somalia deals with its problems. In these talks, there will only be two acceptable answers, ictiraaf from Somalia to SL or SL rejoining. If not, then the answer will probably be made for us, which will harm the sovereignty of ethnic Somalis, regardless of where they are from.