I definitely wouldn't compare myself to either of them. AJR is definitely, to paraphrase Newton, the giant upon whose shoulders I stand, and without his writing there is no way that mine would ever have taken the shape it did. As for Una McCormack, hers is another universe and very worthwhile in its own right. I would not put myself above her, by any means.
I am very glad you like the nonverbal communication. Given that Cardassians can't hear as well as most humanoid races (though it seems the difference is not THAT huge), it stood to reason that sign language would be thought of differently in their culture than it is for species with more humanlike hearing.
Now, AU Skrain doesn't know much of the common sign language that soldiers in both universes are taught--what you're seeing here are his on-the-fly efforts to communicate. He's only had a little basic sign language education--the equivalent of a first-year course, and would not be able to hold his own in a full-fledged conversation.
As to why Garak wouldn't have taught it...well, I think in the main Sigils universe, the Cardassians are very sensitive about that issue and don't like to show just how their culture is different because of their hearing. You speak of ruthlessness...and I would say that's really only the Sigils universe (and it differs from individual to individual). The AU Cardassians may be more stoic than the canon Bajorans, in some ways, but they are of a very different nature.
As for the similarities between him AU and canon Dukat...well, he can
have an acid wit, definitely. And while he definitely does feel his temper flare sometimes, I think he had more control even before this experience than he EVER had in the canon universe as a man almost 50 years older!