These are all valid points, and you are right about extremes. Ed Bearss said there might have been as many Black Confederates as Confederate women who disquised themselves as men to serve in the ranks. IMHO even one percent of the armed soldiers being slaves or FMoC would have noticed and remarked upon by officers and men of both sides in battle reports.
I hate to be a bother, but why would Confederate leaders object to slave or FMoC enrollment if it was already being done, even one percent as you suggest? Elsewhere my question (or one similar to it) is labeled as nonsense, but I hope you see the point. Confederate leaders objected to men of 'mixed blood' being admitted into service, as seen here in correspondence between Maj. Gen. Dabney H. Maury and Secretary of War James H. Seddon --
Note that Maury wants to enroll a single company of Creoles from Mobile, but his concern is that some of them may have "Negro blood." He advises, "They have, many of them, negro blood in the degree which disqualifies other persons of negro race from the rights of citizens, but they do not stand here on the footing of negroes."
Note also Seddon's reply --
Our position with the North and before the world will not allow the employment as armed soldiers of negroes. If these creoles can be naturally and properly discriminated from negroes, the authority may be considered as conferred; otherwise not, unless you can enlist them as "navvies" (to use the English term) or for subordinate working purposes.
J. A. S., Secretary.
In other words, no Negros need apply.