As far as Abraham and the others being monotheists there is a section in Gen where Abraham makes a servent swear an oath if you read it carefully you will notice that it says "swear an oath to the God of heavens and the God of Earth" although not resounding evidence to being multitheistic, it is strange he would not have written it or said it in this way "swear an oath to the God of heavens and earth". It is really not very clear if Abraham was even a real person he may have been, but there is little true evidence to show it.
The first 5 books of the bible seems to have 2 major writers and 2 lesser. The two major writers Were the Yahwest and the Elohist. Much of the differences can be seen in the writings. Take a look at the first 2 chapters of Genisis. They deal with creation, but they are not quite the same even in the order of creation. Many apoligiests have used various explantions for this, but the most common is that they were written by multple people and assembled later in years.
There is a websight that digs into this subject, but unfortunatly I have it bookmarked at work not here at home, I will post it on monday when I go back to work for anyone to read.
Another old religion that is also monotheistic, and you may not beleive it is Hindu. Although they have many many gods theyreally only have one god. That is Brahman "all is Brahman, Brahman is all things. Here is a websight on Hindu beleifs and how much of them are simular to Christianity http://religion-cults.com/Eastern/Hinduism/hindu2.htm It might surprise you.
Also Hinduisem is older then the Hebrew religon, the roots of Hindu are possibly as old as 4000bc, but possibly a later date of 2200bc is proposed. That would make Hindu the first monotheistic beleif. The story of Job is considered to be a Hindu story in it origan, and was adopted into the OT after the contacts with the Persians in the 6th centary BC.
seedy