The question contains a contradiction. In order to actually trace Jews back to Abraham there needs to have been an Abraham. That is the historical issue. I think you are asking a theological issue and this is not the appropriate place for that question. You want to ask Jewish scholars what it means to be Jewish if Abraham did not exist.

(Here is an analogous question where the theological aspect might be clearer. A Christian is someone who was saved by Christ's Sacrifice. But if there was no Adam and Eve what were they saved from? That is a theology question, not a science or history question.)