You are entitled to your opinion. As a Muslim, I feel sorry for those who don't get Islam's message yet believe they do when all they're seeing is the surface.
"Other mammal" here refers to what exactly? As for punishment, that happens when you intentionally deny the existence of the Creator, even when given the access to His word and religion. Islam teaches us that human beings are born pure and good by default, unlike other religions.
Suppose you lived your life on a remote island, never knowing about Islam or the Quran. In that case, you will be treated as if you lived your whole life as a Muslim in the hereafter. Why? Because you did not actively and intentionally deny it, and were not aware of its existence.
If you think religion is childish, OK. I don't.
And was the Quran not a revelation from 'God' only to Muhammad? If he wants to believe it, fine, why should anybody else? Oh yes, eternal torture. It is childish to be told a story and believe it true only because they threaten you.
Yes, the Quran was revealed directly to Muhammad. He told his wife first, and she believed him. It just spread from there. I'd argue that the absolutely miracolous rate at which Islam spread throughout the region is a plus in favor of Islam :)
As for your point about eternal torture, let's say a guy comes up to you in the street nowadays and tells you he's a Prophet sent by God to save humanity. He tells you about this book called Mojimbi which is the word of God. At this point, you have complete freedom to accept of reject his claim. Unless he can convince you that what he's saying is true, you wouldn't believe him and he'd be on his way.
Muhammad however was able to convince people with two things.
One, he was known in his community to be an extremely honest person; never once did anyone hear of him lying. He was a trader, so he dealt with a lot of people. So when he said that he was visited by an angel etc, some people just believed him on the spot. Of course, many others called him crazy, so it wasn't a fool-proof plan.
Two, he had the Quran. Some background first. The community Muhammad lived in was very well-versed in the Arabic language. Most mastered the language and its intricacies by the age of 10, and were known as experts in poetry. Look it up, Arabic is extremely complex.
So he recites some verses from the Quran to people and claims that these are words from Allah (keep in mind he was illiterate). They were amazed, absolutely awestruck by the words coming out of his mouth. The structure and linguistic nature of the verses was perfect, and this was coming from experts in the language! One of the Prophet's biggest critics (and a great poet no less) at the time was almost going to join Islam just because of this.
Now, let's go back to the example. You're a professor of English language at Standford, and a guy you've dealt with a lot and who you know is an extremely honest person in all aspects tells you the same message. He also recites some verses that are so elegant and beautiful in structure that they make Shakespeare look like a pre-schooler's work, and tells you these are the words of God. Wouldn't that make things different?
The second is that no, believing ANYBODY about ANYTHING, no matter how eloquently put, is foolish to say the least. It doesn't make a difference if they are my parent or not, never mind some professor or total stranger. The only person who can decide if something is right or not is YOU. Belief should not be based on how charismatic anyone is, that is irrelevant. Unfortunately, you have total responsibility for your actions and opinions and it is inexcusable to believe anyone or anything without first exploring your doubt. And even then it is up to you to walk the line of what should be believed and what should be questioned.
As for the Quran, it has been passed down by rote memorization. This is the difference between it and other books: people memorized it AND wrote it down. No other book in history has been memorized by so many people at once. Even now the record stands. Of course, the Quran could be falsely memorized in a localized instance, but not in all places at once. Many attempts were made to falsify the Quran systematically, but they failed due to the sheer number of people who had it committed to memory.
I simply don't agree with your opinion on belief.
You've heard of "Chinese Whispers" right?
The apparent beauty of the text says nothing about the truth of the text. Otherwise the King James Bible, considered by most English scholars to be one of the most, if not the most beautiful work of english literature would be entirely true.
I have no opinion on the honesty of Muhammad, it is irrelevant. He might have heard voices, or seen angels and visions on his head that were so real to him as the voice and sight of his own wife. His honesty makes no difference on the truth. Otherwise the visions and revelations of the most honest and righteous Buddhist monks would all be true.
The rate at which an idea spreads has no connection with the truth of the idea. Otherwise Christianity, Mormonism, Hinduism, Buddhism, and even Scientology would all be true.
The truth must stand on its own, in any language, spoken by any or no man. If an idea requires fear and intimidation to make itself true, then it needs no further examination.