Actually, I think that mistake might be more understandable based on what Sanderson's level of comprehension of military culture might be. I always saw Uno as a more of a senior noncomissioned officer, who might in practice be second to the commander in authority and influence, but is junior to actual officers in official command. That would have explained how he and the other soldiers would accept Rand being promoted over him on the Great Hunt operation - Rand is simply another new officer/nobleman who commands, even while the expert soldiers handle the day-to-day stuff. Harnan, in the Band of the Red Hand would be another example, and the Seanchan woman who broke up the troops threatening the menagerie in the beginning of aCoS would be two other examples of the type (Mat noted that he would have easily given the woman a rank equivalent to a modern NCO). People more familiar with popular movies than the minutiae of military command might compare Uno's position to that of Harvey Keitel in "U-571," Robert Patrick in the recent TV show "Last Resort", Sam Elliot in "We Were Soldiers" and Nigel Green in "Zulu". In each case, there are younger and more inexperienced officers who outrank the named character, but officially have command authority over him (Mathew McConaghey, the main female character, Chris Klein & Michael Caine, respectively). This was probably the case with Uno, where he could be comfortable handling a dozen or so soldiers, or when someone else is in charge, like Ingtar, Moiraine or Rand, but it would also explain how dedicated, disciplined professional Shienaran soldiers would let Maseema run wild. Uno lacked real command authority when they were left to their own devices, even he had been the one to enforce discipline when they had a nominal commander. Without an officer or noble and no sense of mission, they felt like they were no longer on duty and let their obediance slip. Another aspect of Moiraine's leadership failures, and why the characters had to move beyond her at the time.

On the other hand, Sanderson did miss the subsequent developments you cited, where he should have moved on and accepted his new role as an officer. Though to be fair, all we have seen him doing are the sort of tasks which can be and frequently are performed by senior NCOs, like leading an escort or training recruits. His officer role is contingent on Egwene's expositional reliability when she identifies Uno as the commander of the army's heavy cavalry - if he is in fact their commander, he should be an officer. Or she might be making the same mistake Rand did back in tGH when he cited Uno as Ingtar's second: in each case a practical assessment of his de facto function overlooks a procedural distinction in the chain of command.

It is still an error of Sanderson's part, but either in his failure to clarify Uno's role and official position or in his failure to take into account his developments off-screen. Since he did the latter with Aviendha (having her suddenly discover issues and problems she had correctly diagnosed and anticipated as far back as book 5) it is very plausible that he might do the same thing with Uno. On the other hand, I kind of had the impression he understood military stuff, so it could simply be his failure to properly convey the character issues too.

It could be simple ignorance, since in KoD, Perrin is expressing doubts about the Cleansing, saying that Grady & Neald claimed it was clean, but he did not see how that could be. Then in tGS, he says to Grady "if you say it's clean, that's good enough for me." This is the same Grady who, in LoC, was talking about going home to his farm that his father was taking care of while he was gone, and in Sanderson's books, became an Asha'man because his now-dead father had been a channeler. It could very well be that Sanderson just skipped a lot of details in his "re-read" before writing.

Which is exactly what some of us WoT nerds are most interested in, hence Sanderson "ruining" the series for us. I think a large part of why aMoL went down so much better than ToM was that, in addition to the heavy action content playing to Sanderson's strengths, we were immunized or callused and had given up on expecting WoT-type writing from these books, so it was able to meet lower expectations.