It is perfectly fine to treat Edwin Betts as a historian, the question from me to you is, was the Historian’s Code of Ethics around when Betts wrote what he did? A lot of what we take for history is say so, such as Tacitus and Suetonius, which goes back a bit further. It is a tenet of mine, that we do not treat people of the past, by the same standards, we treat those of the present. That has nothing to do with withholding information of the past, but it should shine some light on their actions, and how they differ from ours.
As to Jefferson’s great quote, which you gave roughly half of, would change it slightly, to make it more tenable with actual life: “We hold these truths to be self evident, that all men are created equal IN that they are endowed by their creator with certain inalienable rights, amongst which are life, liberty, and the pursuit of happiness.” The capitalized IN is the only change, any others are due to my faulty memory. That was the bridge Jefferson chose not to cross and accept, each and every person’s humanity.
That he was raised in a time when slavery, world wide, was the wallpaper of civilization, does not forgive it, but he does deserve some points for at least showing the right direction forward, if not living up to it.