6. Why should it be necessary for these divine revelations [from different world-religions] - each perfect and adaquate in its way - to have been repeated again and again down the years?

The principle reason for this (apart from other reasons concerning the nature of time and local differences ) is human forgetfulness,
combined with our tendency - as soon as we get our gubby hands on any truth - to bend it, to suit our convenience. The crystal clear stream is polluted in its downward course.

This is what has happened - to a greater or lesser extent - to the previous divine messages.

Islam is final and it is the message of peace.

The inherent saving and transforming power of the Quran is the best proof.