You also don’t have any evidence that directly implicates Brigham Young or the General Authorities of the Church in planning or sanctionning this massacre. It seems as if you are asking to prove a negative. Prove to me that Brigham Young was not involved. Rush loved to say “its precisely because there is no evidence that we must investigate”. Surely you can see this argument is specious, and most of the people that cling to this have axes to grind (see left wing kook fringe).
Of course, one could look at it from the other point... maybe “Plausible deniability” is not a recent invention....