Oh, and before the reveal, I half expected not to see the person’s face and hear the doctor say something along the lines of “The surgery was a success, Mr. Luthor.”
I think AC said something about her teaching him about his origins? If Mera was from where he came from (I think someone mentioned Atlantis, but I don’t remember any fish people in the Pegasus Galaxy 
), she probably didn’t have much experience with surface dwellers until recently. And for all we know, she could have been a priest or something there before meeting AC.
You might be right that it was bad acting, but this is just my take on why it might have been what they intended.
by the way, is this Slade supposed to be the same Slade as Deathstroke (aka Teen Titans)?
The ocean depths get a lot colder than Kansas. 
Nope. Same one.
Although it remind me of General Chang:
I was expecting Lex, but with the “Mirror!” reveal from Jack Nicholson’s Joker.
For a moment I believed she was played by Felicia Day. Funny how a werewolfslayer role can make you think she’ll be in more action series. 