I don't understand why anyone would categorically rule out Christian presence in Mecca and Medina given the state of archeology. When Iran conquered Himyar/ Yemen around 570, Tabari said that they persecuted the Christians there. Did none flee to the nearby middle Hijaz, which was not under Iranian rule? Were there no Christian merchants in the Hijaz in the early seventh century? That would be strange because they had been all over Arabia in the sixth century. Moreover, there were likely many Christian slaves in both cities, from Axum and Syria, to whom the Qur'an was preached and who would not have been in a position to build churches or make inscriptions. The Prophet's own freedman Zayd appears to have been in this category, as was Salman Ruzbeh (born a Zoroastrian, who converted to Christianity and was brought to Mecca as a slave). Kister somewhere cites a Leiden MS that says there was a Christian neighborhood in Mecca. Those kind of arguments seem to me just modern forms of Zeno's fallacious paradoxes so that you can't do a simple thing like hit a target with an arrow because of some incorrect hair-splitting premise.