The Argument for Islam is very Simple:

[Crunchy]

The history of Saudi Arabia shows that "someone" conquered the place around 600AD and that "someone" spread islam around that time.

There were always warring tribes.  So what?  The emergence of islam is a far more complicated process than the assholes finally wrote down in their silly book.  In much the same way as the emergence of jesusism was a long, slow, complicated process that had more to do with imperial power than any fucking "god."

Again, I don't see the point.
Muslims believe he was real (as do the vast majority of historians), so what's wrong with pointing out his horrific words and deeds that we get from their own holy texts? In his own words and the words of his trusted companions, Muhammad was a violently xenophobic, violently misogynistic murderer, rapist and pedophile and these ideas make up a great deal of his teachings. That is all the argument against islam that is needed.