Gabriel is the official messenger of the God of Adam, Abraham, Moses, etc. So, if you are going to base the new religion on the traditions of the old, then it is necessary to have Gabriel in the mix. Seeing him wherever you look is just a method of making the event seem supernatural.
Did Mohammed make it up, or was it a delusion? That's a tough question to answer. But I think he made it up. Evidence for this? Not much, it's mostly opinion.
One theory on motivation is that Mohammed, seeing the power of the Christian empire to the north, decided that the Arabs would do better with a united religion rather than hundreds of Gods.
There has to have been some research, the follow-on from the previous faiths cannot be coincidence. The Quran includes stories from one of the Gospels of Thomas, I don't know how well known those stories were in the region. But they're not mainstream Christianity today.
When the locals tried to test Mohammed, they would go to the Jews in order to get a really tough question from their scriptures. Mohammed would then have to "wait for a revelation" in order to get the answer. Which indicates to me that the answer was based on research, rather than delusion.
Then there is the change in direction of prayer, from Jerusalem to Mecca.
http://www.sacred-texts.com/isl/pick/002.htm
142 The foolish of the people will say: What hath turned them from the qiblah which they formerly observed ? Say: Unto Allah belong the East and the West. He guideth whom He will unto a straight path.
143 Thus We have appointed you a middle nation, that ye may be witnesses against mankind, and that the messenger may be a witness against you. And We appointed the qiblah which ye formerly observed only that We might know him who followeth the messenger, from him who turneth on his heels. In truth it was a hard (test) save for those whom Allah guided. But it was not Allah's purpose that your faith should be in vain, for Allah is Full of Pity, Merciful toward mankind.
144 We have seen the turning of thy face to heaven (for guidance, O Muhammad). And now verily We shall make thee turn (in prayer) toward a qiblah which is dear to thee. So turn thy face toward the Inviolable Place of Worship, and ye (O Muslims), wheresoever ye may be, turn your faces (when ye pray) toward it. Lo! Those who have received the Scripture know that (this revelation) is the Truth from their Lord. And Allah is not unaware of what they do.
So the reasoning seems to be:
Jerusalem was given as the first direction so that God could tell the difference between Pagans and Muslims. And the reason for changing is because Mecca was dear to Mohammed.
I think it was because the Muslims' main enemy in Arabia were the people who profited form Mecca being the centre of religion in the area. By turning to Jerusalem to worship a God who is supposed to be everywhere, the Muslims were threatening Mecca's livelihood.
The Qaba was associated with Abraham before Mohammed left Mecca. But there was no attempt to mark it as a place of worship for Muslims. Sometimes, the presence of the idols is used as an explanation for why the Qaba wasn't a place of worhip for Muslims. But the Muslims tried to make a pilgrimage to Mecca (from Medina). They were stopped. A treaty was signed allowing them to make a pilgimage the following year. I don't think there is anything in that agreement which would require the Meccans to remove the idols from the Qaba while it was being used by the Muslims.
Khalid bin Walid was one of the most effective generals that the Quraysh had -- he had defeated the Muslims in the Battle of Uhud. But, after that above treaty was signed, he converted to Islam. Did he see the light? Or did he realise that it was easier to keep Mecca the centre of religion by making it the centre of the new religion?
The Quran itself admits that the pagans performed some of the rituals Muslims are permitted to perform:
http://www.sacred-texts.com/isl/pick/002.htm
158 Lo! (the mountains) As-Safa and Al-Marwah are among the indications of Allah. It is therefore no sin for him who is on pilgrimage to the House (of God) or visiteth it, to go around them (as the pagan custom is). And he who doeth good of his own accord, (for him) lo! Allah is Responsive, Aware.
Which reassures the Muslims that it is okay to do as the Pagans did. Sura 2 is mostly an early Medina Sura. Which adds to my guess that Mohammed did not tell Muslims to perform these rituals from the beginning. I think they are being added later, in order to undermine opposition from those that make their living out of Mecca being the centre of religion. If Mecca still has a livelihood under Islam, then why should those that lead Mecca have cause to wage war with Muslims?
As I said,
That's mostly opinion. But it's why I think Mohammed planned things out rather than simply remember delusions.
