Islam critiqued cannot find historical precedent; early marriages an accepted Islamic norm?

In answer to the video "Surah 65:4 and Child Marriage in Islam"

Public criticism, according to the tradition, was laid at Umar for his trying to marry Ali's daughter, because she was young and Umar was old, almost the same age difference between the Prophet and Aisha. In fact, the traditions record that this was one of the objections of Ali for the union and public criticism had gotten to the point Umar had to defend himself for the move in public. Umar made the point that he only wanted to marry her to affirm his ties with the House of the Prophet. Umar didnt use the precedent of the Prophet marrying Aisha at an alleged young age. This incident, involving the closest companions of the prophet, reveals the same inconsistencies as those noted earlier with the prophet's refusal to marry his daughter to AbuBakr. 

Also, if it was a norm of Arab culture, then why would he draw the criticism of the city of Medina for such an act?

Further, in the Muwatta which is the representative work on the school of Medina, in the chapter on marrying younger women without asking them, this narration isnt even reported. And you would think, in Medina of all places, the marriage of the Prophet to a young Aisha would serve an evidence for the people of Medina.

Where are the examples to substantiate the point of view that such practice was a norm in Arab culture?

An interesting point to re-stress is that not only are there no examples to show how such practice was a norm, but also Urwa, the ultimate source of these traditions regarding Aisha's age, just so happenned to have married a 9year old.

Besides his poor memory at the time he reported Aisha's age, there is now another reason to seriously doubt his credibility, namely the blatant need to falsify a hadith to serve his interest in justifying an act that far from being the "norm" of the prophet's time, was unacceptable to many including the prophet's own entourage.