In response to your post and questions, Johnpowell, I would like to try and clarify somethings that you might have never heard of or might have ignored.
In your post you mentioned or rather asked a few questions like "Did he understand that the Earth is round, how old the Earth is, what fuels the stars, where the chemical elements came from, how we evolved from the apes, how oil formed, etc.?"
My friend it has been mentioned in the Holy Quran, which was sent down to us Muslims thousands of years ago, that the Earth is round, and that the planets revolve around the Sun. This is a fact and this is what Muslims believed in even thousands of years ago when all other religions believed that the Earth was flat. Some even believed that the Earth was carried on the back of a giant turtle. But we Muslims knew the truth.
Another fact that I am particularly proud of is that the Quran mentions that the Sun revolves around its own axis. And it is just recently that the scientists discovered this.
A thousand years ago, the Quran mentioned the smallest atom. When the rest of the mankind only discovered it so many hundreds of years later.
You should understand that Prophet Muhammed (SAWS), was not the 'inventor' of our religion. We believe that the Quran is a devine revelation of Allah (SWT), just like we believe that the Bible is a devine revelation.
Dear John, to understand the depths of our religion, you will have to do a lot more research than this. Islam is very easy and yet so difficult to understand.
Coming back to the subject, we believe that Jinns are beings made of smoke and that they live their lives similar to us, they share this earth with us. They live, die, procreate just like us.
You said "If the Jinn are invisible, then they might as well not exist." John, it is not upto us to understand these things. No one can question or understand the reasons. Jinns probably serve a purpose just as everything else in this world serves a purpose, from the ant to the elephant. Believe me, nothing here exists without having a purpose.
I hope you have understood what I had to say. And I hope that you would try to do more research to understand our religion.
Everything is mentioned in the Quran, look for it and you will find it. It might be right under you nose but you may miss it.