Abraham is regarded as the father of the Jewish religion. There is some scholarly discussion indicating that he is the father of Judaism, Islam and Christianity. Christianity would not exist if not for Judaism, and there can be no debate that Judaism is the older religion. While some scholars present Abraham as Muslim prophet, I believe that the religion of Abraham is Judaism.
Judaism does trace back to Abraham and should be considered the pure religion of Abraham. It should be noted though that in many contexts the Jews don't remain as god's people over time. So yes going back far enough Abraham was a Jew but over time the Jews lost favor.
I'm not quite sure what you mean by the 'pure' religion of Abraham, but Judaism is the religion that holds Abraham to be the last prophet. They are then waiting for the next prophet, which they do not believe was Jesus Christ. Jews do hold up Abraham as a significant historical figure.
I believe Judaism is the pure religion of Abraham. Given he wasn't really seeking out religion and it found him, I don't think he had much of a choice, but overall I don't see any other religion he could be directly attributed to. Do he create it on his own, no I don't believe so.
A case could certainly be made that Judaism is the original, since it is, after all, the religion of Abraham, himself, and from where Christianity and Islam descend. Still, saying something is pure implies that it is better, or superior, something that is not the case - no religion is above any.