Bruce, I wonder if skimming through the Daily Missouri Democrat for April and May 1864 would be productive. The Democrat, being a Radical publication, may have weighed in on the matter. Perhaps published the order, and commented on its effect. I would be surprised if such a far-reaching order that gutted the EMM would have resulted in silence from a publication that championed the politics of those who were targeted by the order. Also probably wouldn't hurt to look at the Daily Missouri Republican and the St. Louis Union for the same time period. I have found that the Union sometimes published stories that the others wouldn't touch (ex.--the mutinies in the PEMM).
And I wonder whether the special order targeted just officers, or also included enlisted men. Given the scores of officers targeted, if enlisted men were also included the total number of men dismissed would have had to have been breathtaking (which also may have merited mention in the Radical press). The word bloodbath would be used in our present times for a political purge on this scale.
Also, in my audit of the long list of officers targeted for dismissal by S.O. 126, a very significant portion had German names. I don't really know what to make of that, but think that it bears mentioning.
And back to what brought us here in the first place--was David Guitar a Confederate sympathizer, or secret sympathizer? I think that it is beginning to look like not only was he not a Confederate sympathizer, but that to classify him as being one just might be akin to classifying Ted Nugent as being a PETA sympathizer....