There is no Islamic Empire.
The title of the Islamic Empire was Caliphate. The Caliph was called Amir-ul-Momineen.
It depends on the Islamic Empire in question. Assuming that you are talking about the Abbassid Caliphate (which is most commonly referred to as "the Islamic Empire"), the group that brought them down was the MONGOLS.
In 1492 the Islamic Empire finally left Spain but i dont know when it began.
what empire
Yes. The Mughal Empire was a great Empire of the Muslims in India.
Different in different parts of the empire.
Ottoman Empire
The Abbasids, as the ones from the Islamic Empire.
Invasions by the Islamic Ottoman Empire destroyed the Byzantine Empire.
yes
By forming Islamic armies, conquering non-Islamic countries, and making them into Islamic countries. Simple.