I think the point is, though, that there are/were several different kinds of uniquely Mormon blood atonement (aside from the universal Christian idea of Christ's blood atonement), and it's not as easy as saying blood atonement was or wasn't part of the early church, because there were differing definitions.
The period article highlights two obvious ones: blood atonement (i.e. capital punishment by firing squad) for legally convicted murders, vs. blood atonement by killing those who commit the "sin" of apostacizing from the church.
The period article says the first definitely
is church doctrine (at that time), while the second
isn't.
Each of the links you posted addresses at least some of the different definitions, but like a lot on this topic, they seem to mix and match the various meanings.
There are also other nuances, such as whether the church believed they had the authority to kill someone on their own, whether the church could only condemn a person through secular law, whether the person must kill themselves, what crimes/sins are subject to blood atonement (one of your links mentions an example where it was used for adultery--not surprising in an era when rape was also a capital crime, but not the first thing one thinks of concerning blood atonement).
To show the long-lasting echoes of the version of blood atonement where it's a form of capital punishment after legal conviction of murder, death by firing squad was a choice in Utah up until recently, and there's the example of convicted murderer Ronnie Lee Gardner:
http://en.wikipedia.org/wiki/Execution_by_firing_squad
From the Deseret News
article linked in the Wikipedia footnote:

Protestant/Catholic Christians don't seem to be embarrassed over Leviticus, and this seems no worse than anything in there.
I wish Skyrider44 would come back and expound further on the
active LDS he mentioned who consider the Book of Mormon "a work of the 19th century."