Let me clarify what you mean;
There were multiple prophets, but the prophets who came before Muhammed were not able to effectively transmit Allah's message. They either misunderstood God, or they understood him but were in turn misunderstood. Is that right?
And so Allah sent another prophet, and declared him the final prophet, since he knew Muhammed would spread the true message.
Allah is all-knowing, so he knew before he gave his message to earlier prophets, such as Jesus, that the message would not be spread correctly.
Is that what you mean?
As for the second part of the equation (Islam in pre-Muhammed times) why, in your estimation, is there no mention of Islam in any documents anywhere on earth from those times? I can ignore, for now, the Quran being assembled post-Muhammed.