Well put, Ralph. Yes, we always have to "allow" the author's opinions and conclusions. Our challenge is to make our best judgement as to whether the author arrives at those opinions and conclusions based on reasonable and sufficient available sources and whether the author is promoting a particular agenda to the extent that they are leaving out sources that are counter to their personal "agenda".
In the case of Rampp & Rampp, it would appear that they are so unfamiliar with the subject matter they made the simplest of mistakes and which then snowball in their effect.
In the case of Annie Abel, she had done much scholarly study on the subject of US-Indian relations in the 1800s, she located and used an impressive amount of documentation/correspondence of the Indian Office around and during the war, and she relied on the ORs, Confederate Records, and other War Dept sources. If she had a personal agenda, I believe that agenda was to expose the graft and corruption of the US Government, politicians, and others involved with the Indians during the period. She does, of course, have personal biases that come through in her books. She does have a pro-Union point of view. She does have racial biases regarding Indians and Blacks. All things considered, it is my opinion that she very well supports her opinions and conclusions, whether I agree with them or not, and that her biases do not unreasonab ly taint her presentation and consideration of the facts available. I assume that her perception of the Choctaw during the war had more than a little to do with Ms. Wright's dislike of her work.
My father was a fundamentalist Christian minister and, regarding the understanding of the scriptures, he often commented that you have to consider who was speaking, to whom were they speaking, and why were they saying what they were saying. That advice comes to my mind often. I love reading Wiley Britton but it is important to understand who he was, who he perceived his audience to be, and what point he was trying to make. From him we have an invaluable first person account, understanding that it was written (in the case of the Memiors written in 1863) by a 21 year old who was distinctly abolisionist and pro-Union and distinctly proud of his comrades in the Kansas regiments and of some of the prominent officers (particularly Wm A Phillips). It seems apparent to me that it is one of his intents to correct the view that the Kansans were inclined to jayhawking and otherwise mistreated civilians, primarily but not exclusively those her were pro-Southern -- though in my opinion he is unsuccessful in this regard and comes across in rationalizing and discounting the acts of "Northern" jayhawkers while exaggerating the acts of "Southern" bushwackers.
I would go on to say that regarding the ORs and similar sources, we must keep in mind who, to whom, and why the officer or other person was writing. By doing so, we are hopefully less inclined to be misled by reports that exaggerate the writer's role and accomplishments. We are also more wary of their knowledge of the facts around their topic -- e.g. Britton presents his own observations and those he acquired from others but was, of course, rarely informed of all the facts around particular events such as knowledge of the enemy.
One closing comment on Annie Abel's books, I beleive they are not as popular with many readers on the subject because they do not contain specifics of battles, opposing forces, and the like.