I think, for me, the biggest thing in the possibility that Genny was Devastation was that it wouldn't be a new enemy. That theory was a neat two-birds-with-one-stone - it retrofits a crappy, problematic old enemy (Deva as a meaningful foe for Diana is simply flawed at its most basic level) into something more usable and worthwhile, and it neatly sidesteps Wonder Woman Syndrome.
Because in most franchises, new bad guys, new allies, new ideas and characters are a good thing, difficult to come by and something you have to actively encourage in writers (everyone wants to do the definitive Joker story instead). But in the Wondy franchise, the opposite is true - there's no continuity, no carryover, no respect or work given to existing supporting cast or foes, and the hardest damn thing in the world to get a writer to do is not drown the title in All New Everything. Every time I see a new character introduced to the Wondy mythos, I flinch - just one more character for the next writer to ignore and replace with his own. Now that Genocide is back to that, she loses a lot of the points I had subconsciously given her when I suspected she was Deva.
when did Wacky Alky learn about the Bana?
I can see Polly telling them. Her little "do you repent" business is an annual tradition; supposing it fell during the first civil war, she might have shown up bloody or late and provoked Wacky Alky's curiosity. Fighting might have passed by their cells during the second civil war, as well. (Both possibilities would give her good reason to dislike the Bana.)
Or perhaps that Themysciran who betrayed everybody during the second civil war was imprisoned near one of them - she was dating a Bana, and the Bana are why she got locked up, I can see the question coming up - or perhaps the Circle were freed along with everybody else during the "all hands on deck" to fight Apokolips that got said Themysciran out of prison.
(This is why "there all along" retcons to Themysciran culture are somewhat problematic.)