Brother Abdusamad, to answer your question of why a person asking for help from the dead would be considered shirk, eventhough in his heart he believes Allah (swt) is the supreme being, the reasoning is we muslims do not know what's in a person's heart nor can we ascentuate sincerity of his intentions. We only judge by whats apparent, (ie the actions of the individual in question). Therefore if a person "claims" he "really" believes only Allah can accept his dua, then he should stop praying to others.
Also this line of reasoning (the use of intercessions) has been attributed to the mushrikeen, the christians, in many verses of the Quran.
In addition, the prophets of Allah (swt) were sent to mankind, not that they make people believe their is a God (Ilah), but that they call to the WORSHIP of the one and only ALLAH. This is the message of Islam, not only to acknowledge Allah's soverignty, but to WORSHIP him alone. This is the essence of Sura Fatiha, and the entire message of Islam.