The Last Templar: How much does historical fiction need to stick to history?

Raymond Khoury's work of historical fiction, The Last Templar, like Dan Brown's The da Vinci Code incorporates historical facts as arguments to support theories within the novel. The fictional characters of the book cannot be held accountable for the facts they present because they are by nature fictional. Yet it would seem that a fictional character who is an academic scholar (specifically one of the antagonists: Vance) should have their general facts straight. On several points in the book I was surprised to find that Vance had simple facts wrong. I don't know whether this is the result of poor research on the part of Khoury or the fact that Khoury didn't feel that his characters needed to stick to facts in certain contexts of the story. 

A very clear example of misinformation is Vance's account of how Hagar was banished.  

"... Abraham's wife, Sarah, couldn't have children, so he took a second wife, his Arab maidservant Hagar, who gave him a son they called Ishmael. Thirteen years later, Sarah manages to have a son, Isaac. Abraham dies, Sarah banishes Hagar and Ishmael, and the Semitic race is split between Arab and Jew." (The Last Templar, Chapter 68, page 327)

First of all, Hagar was an Egyptian (Genesis 16:1). Secondly the biblical account is that Abraham was the one to banish Hagar and Ishmael on Sarah's request, obviously while he was living (Genesis 21:14). These are not significant points upon which key arguments hinge but they are annoying to find these kinds of errors in historical fiction. I wonder if Khoury was using a Islamic account of this event, instead of a Judeo-Christian one. Is this sort of thing common in other historical fiction? I am not always knowledgeable in the culture, history and setting of other historical fiction I read. It could be that authors of historical fiction cut these kinds of corners all the time, and I have been oblivious all this time.