The Secret Six seldom actually has six people. It's kind of a running gag. And at least "seven" also starts with an "s," unlike "five." ^^
The thing with Kay seems a lot more definitive than the Batgirl tease, though. The clues are better. Consider: the first we hear of any of it is with the scene where we first meet Liana. Scandal has some kind of weird fugue where she perceives this random stripper-for-hire as her actual girlfriend; fugue!Knockout says "I might be a drunken delusion, or maybe, I'm actually Kay speaking through this woman because I'm a Goddamn New God and I spit in the face of death."
Second clue was the waiter in Jeanette's place: a total stranger who leaned down and warned Scandal of the impending attack in intimate terms, and then a moment later seemed like a completely different person who had no knowledge whatsoever of that warning.
With most other characters this would be meaningless, but that leads us to the third clue: this is all happening relatively concurrent with FINAL CRISIS, wherein Darkseid and his Apolalyptian ilk were bouncing around and possessing human bodies after their prior forms were taken out by the New God killer.
Knockout being one of said ilk, slain in the same way... it would not be especially difficult to believe that she simply chose not to be part of Darkseid's nonsense or recorporate with the rest of the New Gods at the end of FC, and instead is staying by her lover's side. My assumption is that Liana is an entirely third party, but Kay may be hitching a ride with her at the moment and subtly influencing her, or have left some influence on her when she was in control before (provided that was a legit possession).