Firstly, Arabs were the majority. Don't confuse religious groups with ethnic groups - the majority of Shi'a were Arabs, mostly centred in the south, and the Jews, who numbered around 100,000 in Baghdad, were all Arabs and early proponents of Arab nationalism. Kurds made around 20% of the population, and then you could break them down into Sunni, Shi'a, Christian and Yazidi. There were also Turkomen and Iranians, though they are smaller minorities (too small to carry much political weight).

Faisal was an Arab Sunni from a foreign land (Hejaz), and his natural allies were the Arab Sunni elites. But they represented a small minority. Pan-Arabism (which Faisal was already partial towards and an early proponent of) was adopted as the state's ideology because it crossed religious boundaries, and could be seen as a way to bridge the gaps between Shia, Sunni, Jew and Christian.

I don't understand what you mean when you say Iraq never was able to unify. It was formed territorially by the three ex-Ottoman wilayets of Baghdad, Basra and Mosul, but the British and then Faisal both faced serious internal threats. In the south, there was an Arab revolt (led by Shi'a if I remember correctly). In the north, there were multiple Kurdish breakaway movements. Turkey claimed Mosul as their's and were engaged in some limited military operations there.

By the end of the 1920s, Turkey had dropped its territorial claims and the League of Nations had established Iraq's borders. The Kurds were subdued, at least temporarily, with promises of respect for their minority rights - though these promises turned out empty, they dispelled growing resentment against the Baghdad government.

When Faisal died in 1933, I'd say that his nation-building had been relatively successful, at least in terms of the stability of Iraq. The Arab nationalist ideology was very successfully implemented in the schools (you mention Sati' al-Husri) and in the army. Young Iraqis were being indoctrinated in pan-arabism.

I apologise, this answer has been a bit rushed and rough on the details. Like I say I don't know what you mean by the failure to unify.

Some interesting articles: Michael Eppel, The Elite, the Effendiyya, and the Growth of Nationalism and Pan-Arabism in Hashemite Iraq, 1921-1958, International Journal of Middle East Studies, Vol. 30, No. 2 (May, 1998), pp. 227-250

Reeva S. Simon, The Teaching of History in Iraq before the Rashid Ali Coup of 1941, Middle Eastern Studies, Vol. 22, No. 1 (Jan., 1986), pp. 37-51

I will try to post more later.

I'm getting my timelines messed up. In my notes I have when Saddam took power in Iraq, 90% were Persian/Jew. My professor also said Faisal rejected Husri's ideology of Arab Nationalism after he had recruited Husri to write the textbooks for Iraqi schools and that Faisal viewed Husri's ideas as romantic poetry. And I used the wrong word when I said unify. I don't even know what I'm trying to ask, this is very confusing for me.