Liz Taylor's coming back to town, at last. She's absolutely sworn to zip into the big black-tie World Premiere of "Genocide" at the KenCen next Sunday. (Liz, along with Orson Welles, narrated the new documentary on the Holocaust; neither charged a nickel.) Frank Sinatra's Chair of the evening; Nazi-hunter Simon Wiesenthal will show; and Abe and Irene Pollin -- he owns the Cap Center, and so much more -- are the grand hosts for the premiere and the party. Not a freebie, natch. But the Invite's worded in Terrific Taste: "Couvert: $500 per person." (It all goes to the Simon Wiesenthal Center, you know.) Word's around that Liz and John will probably roll in a deux. Excitement runs high. Ear is definitely to the ground . . . And perkier-since-the-pokey Marvin Mandel and the fair Jeanne lovey-dovied at Mel's last week, talking turkey with Renee Poussaint for a teevee special. The murmur went around: Bootsie, the ex-Mrs. M., was in town; she had been spotted sashaying into Mel's. But no. It was a spiffily frocked lookalike. Bootsie munches at Pisces and the Palm these days. "A narrow miss," muttered Mel. Restaurateurs simply live in dread, you know.