I've spotted "William" in O'Dell's account. Still searching for it in Bailey's.

Here's what intrigues me, and points to Jim Reed, a Quantrill Raider whose family had ties to the Order 11 burnt district, as being "Reed". This from the Council Grove Republican, March 20 1891, p. 8. In speaking of Jim and Bill Anderson, John Maloy states "They were, in fact, notwithstanding the general laxity of those days, considered outlaws, and the whole community was kept in a state of terror through their intimidation and threats. About this time the Anderson boys had associated with them two others, Lee Griffin, and one Reed, who afterwards became one of the husbands of the notorious Belle Star, who was killed in the Indian Territory a couple of years since...."

This tags "one Reed" as being Quantrill's man, Jim Reed, who isn't some nebulous mystery man, and is pretty easy to get a handle on that puts him in very close proximity to the Andersons. In addition, in regard to the killing of Baker most accounts speak of Bill Anderson, Jim Anderson, Lee Griffin, Reed, and "an unidentified fifth man." I have yet another source identifying that fifth man as Dick Yeager, which ties it all together pretty snugly, given that ten months later Bill Anderson and Dick Yeager led the famous raid into the Council Grove area that was the precursor to the Lawrence Massacre raid two months after that.