Muhammad's errors

A regular reader of Rational Islam? recently sent me the following regarding his discussions with some Islamic experts (I have his permission to quote from his mail):
...I've even personally gone to a hafiz and a sheikh to discuss my concerns and I've seen exactly the irrational mindset they defend their arguments (Satan must be whispering to you/You are being tested/You are hanging around with the wrong people/You must learn arabic yourself etc) these are exactly the convenient excuses which trap the mind from reasonable thinking. Even when they try to provide a rational answer to some absurdities translations need to be changed or there are hidden meanings we do not know yet.
Anyway I have been trying to explain this to my Muslim friends, but they won't accept it because of the following major issues:
- "Then why did the Prophet do everything he did?" ("He went through so much to spread a message - surely it was divine")
- "How could he have copied from Jewish/Christian sources. What he preached was different and didn't contain errors that the previous did therefore he must be right"...
There are two separate issues here.
Firstly: Why did Muhammad devote his life to spreading his message if he wasn't divine?
I shall ignore the obvious (to non-believers) answer that Muhammad had plenty to gain from "going through so much to spread his message": wealth, power and sex to name but three. (There are, after all, many verses in the Qur'an that give special privileges to Islam's founder: 8:1 springs immediately to mind - They will ask thee about the spoils of war. Say: "All spoils of war belong to God and the Apostle."...Was Attila the Hun divine because he dedicated his life to achieving his power and wealth? Was Alexander the Great? A single-minded determination to achieve one's ends and a willingness to self-sacrifice is no guarantee of divinity - especially if that supposed self-sacrifice leads to abundant riches, political power and a constant supply of female captives!)
Let us consider instead the fact that the question presupposes that if someone genuinely believes in their calling and is prepared to sacrifice everything to its end, then it is impossible for them to be mistaken. It thus ignores the possibility of sincere error. Why, we must ask ourselves, do Muslims discount the possibility that Muhammad was suffering from some sort of pathological delusion. What in the Prophet's behaviour or message proves he was saner that anyone else who claims to have heard voices from an angel/God?

Secondly and more interestingly we are asked to consider: How could Muhammad have copied from Jewish/Christian sources when what he preached was different and didn't contain the errors that were in the previous scriptures.
Do the differences seen in the Qur'an correct mistakes? Otherwise they are simply differences and prove nothing.
Muhammad did indeed change some details of the well-known biblical stories - such as the Ark coming to rest on Mount Judi instead of Mount Ararat or God saving "Pharaoh"'s body or, more infamously, a replacement being crucified instead of Jesus. But why do these differences make Muhammad's message any more credible than the Bible? There is no proof, after all for any element of any of these stories. We're still expected to swallow the preposterous tales from the Old Testament. As for the New Testament differences, do Muslims really believe that having Jesus speaking from the cradle and making clay birds come to life is more credible than stories about the Jewish agitator in the Canonical Gospels? (And of course in this latter case Muhammad didn't change the stories but simply copied from the non-canonical Gospels which also include, among other bizarre tales, stories of dragons.)
Without knowing what "errors" are being referred to it's of course difficult to counter this with any thoroughness, but straight off the bat it is not difficult to see why a text written by / revealed to one man should contain fewer immediately obvious contradictions/mistakes than one written by many authors at different times and collected over a long period (as were the Old and New Testaments and the purely Jewish texts).
Nonetheless, it is not difficult to find anomalies and inconsistencies in the Qur'an's recycling of the previous scriptures.
Here's an obvious example that no-one has been able to explain to me. The following two verses from the Qur'an describe Mary, mother of Jesus.
And Mary the daughter of 'Imran, who guarded her chastity; and We breathed into (her body) of Our spirit; and she testified to the truth of the words of her Lord and of His Revelations, and was one of the devout (servants).66:12
At length she brought the (babe) to her people, carrying him (in her arms). They said: "O Mary! truly an amazing thing hast thou brought!O sister of Aaron! Thy father was not a man of evil, nor thy mother a woman unchaste!" 19:27-28 
 It becomes immediately clear that there is a problem here. Mary, the mother of Jesus, is certainly not the sister of Aaron. Nor is she the daughter of Imram
But there is someone in the Bible who is. Her name is Miriam.
Here's how Miriam is described in the Bible:
Now the name of Amram’s wife was Jochebed, daughter of Levi, who was born to Levi in Egypt. And to Amram she bore Aaron, Moses, and Miriam their sister. (Numbers 26:59 NET Bible)Miriam the prophetess, the sister of Aaron, took a hand-drum in her hand, and all the women went out after her with hand-drums and with dances. (Exodus 15:20 NET Bible)
Why might Muhammad have confused the two? Because in Arabic the names Mary and Miriam are rendered the same.

This mistake was even noticed by the Arabic Christians of Najran.  How do we know this? Because there is a record of them exposing Muhammad's mistake about Mary in the reliable hadith of "Sahih Muslim", (considered to be the 3rd most important set of books in Islam after the Quran, and the Bukhari hadith.  
In Sahih Muslim, the hadith related by Mughirah ibn Shu'bah, #5326, says:

            "When I came to Najran, they (the Christians of Najran) asked me:  You read "Sister of Harun", (i.e. Mary), in the Qur'an, whereas Moses was born well before Jesus. When I came back to Allah's Messenger I asked him about that, and he said:  "The (people of the old age) used to give names (to their persons) after the names of Apostle and pious persons who had gone before them.""  [Sahih Muslim, translated by Abdul Siddiqi].
Three things have always struck me about this "explanation". 
i. Muhammad is able to explain without having to receive a revelation from Allah. Did the Prophet know Allah's thoughts, then?
ii. The people of old age did occasionally give names after the names of pious persons who had gone before but this was son/daughter (not sister/brother) and the pious person would be chosen to make the greatest impact. So why choose Aaron and not Moses and why say sister not daughter?
iii. Why did no-one, not the Muslim who was asked, nor the Arab Christians, know about this custom? Why should God have made a reference which was evidently so esoteric as to be understood by no-one who heard it?
iv. And the clincher... why is Mary referred to not only as Aaron's sister but ALSO as Imram's daughter? 
(Remember) when the wife of 'Imran said: My Lord I have vowed unto Thee that which is in my belly as a consecrated (offering). Accept it from me. Lo! Thou, only Thou, art the Hearer, the Knower! 3:35
Using Occam's Razor to slice away the myriad  contortions needed to accept the Muslim explanation, we are left with the simplest answer : the writer of the Qur'an made a mistake. Ergo the Qur'an is of human origin.