Prophet Muhammad saws launched the Tabuk campaign against the Orthodox Christians in his lifetime (9 AH.) That and the solitary shahadat of Hazrat Ayyub al Ansari raa at the gates of Constantinople proves without a doubt that the Turkish armies which assailed Constantinople and the ghazwa which saw the city eventually taken over by Sultan Mehmet el Fatih ra were blessed by Allah swt.
Throughout that whole time, not one church of the Christians was turned into a masjid, except the royal church of the enemy Byzantines, which was the symbol of their power and influence. It was more of a political gesture, along with the change of the name of the city.
The word Istanbul or Istambul comes from the Turkified name for Constantinople, so its not much different.
Regarding the Turks taking Christian ruling family noble boys from their families, this is another exaggeration by the Sheikh. Those boys were taken with full consent of their families to be raised as Janissaries/Yeñiçeriler in the royal guard of the Sultan of the Ottoman Empire. They wielded considerable political, economic, and military power in the empire and became advisors, governors, and generals of the army. This was done to create an army of faithful loyal soldiers, which did not have ties to rival Turkish or Muslim tribes, hence ensuring longevity to the state.
However like the Hindus, the Orthodox Christians under Russia, Serbia, and Greece have created a parallel history embellished with falsehoods, lies, exaggerations to show the Turks as barbaric, evil, and villainous people worthy of being slaughtered and erased from the world. At times this mindset influences state policy of these countries, like the massacres during Russo-Turkish war, Graeco-Turkish war, and the breakdown of Yugoslavia leading to the subsequent massacres of Kosovars and Bosnians.
