1) Quran was organised into suras and written down during Umar r.a. and Uthman r.a. They gathered the written pieces. But there were also many memorizers of the Scripture by heart, who were tutored by Muhammad pbuh. No other book of that volume can be memorized word by word. A collective and transparent work was done. Not like a Gospel author writing on his own.
In short, Qur'an is not inspiration. It is revealation. If inspiration is the correct word for the OT and NT why not follow Dalai Lama too. He is really inspired guy. How can one tell Paul's inspiration is divine but Dalai Lama's is not? What makes the difference?
2) Muhammad pbuh is sinless like other prophets, in the sense that His sins are purified by God, since prophets are sent to be a model for believers.
He always asked for forgiveness of God because there is always more to do to please Him. There are always further steps to take toward God. No human being can praise God, and acknowledge His Might as proper as He deserves.
3) Christians make many claims about Qur'an, which if they think wise and unbounded would lead them to rethink about their books. They cannot solve discrepancies between two verses in the Bible, but claim there's discrepancy between Uzair in the Qur'an and Jewish Ezra. The fact that Jews now do not believe such a thing now, does not mean they did not make that claim in the past. It is that simple.
Christians are naively remodelling scientific theories to put it in line with the claim that the Bible states the Earth, even Universe, is some thousand years old. Google the words young+universe+bible. A commentary even claimed that God created Universe already (several billion years) aged!
But they cannot think of simpler and straightforward commentaries of Qur'an verses. That is because they have conditioned themselves to conceal the truth.