If they reject the Hebrew god, then they would also reject Abraham.
No. If they reject the Hebrew god, then they do not reject the god of Abraham bu the god of Moses (even if they are one adn the same but that also includes the Christian god).
But this is becoming more and more off topic...
Back to topic:
I really doubt that Islam would have survived if Muhammad was killed before he took Mecca, because the military successes of Muhammad made Islam interesting. And then there is also the Islamic believe that Allah protects his prophet from any harm (that's the reason they believe that a double was cricified instead of Jesus Christ) and a non-natural death would have shattered this believe.
But this is all speculation based on the truthfulness of the Islamic tradition.