... don't think he's in any of the costumes, and I'll explain why in a moment). During his ominous walk-up following the laugh, you can see the back and rump of the panda costume on the left of the screen, still with its rifle strap, and moving organically. Further, your speculation that the tattoo'd man holding Harleen's head down is Joker is almost certainly correct, as the still you pulled also clearly shows the purple glove on his right hand (desaturated, like the whole DCCU, but you can also see his bare left thumb, pasty complexion, and playing cards tattoo). Due to both of these, however, the Panda/Batman attack was most likely perpetrated by Joker's goons to free him from whatever facility (probably Arkham) he was being held in, which culminated in the torture of Dr. Quinzel and the birth of Harley Quinn, and therefore could not be the scene which leads to Batman chasing Harley and Mistah J (she already has her Harley look, including multiple facial tattoos - you know they're tattoos because she also has them in prison - during the car scene). Based on the set leaks - the pictures of Joker confronting Harley in front of the sports car and her holding a gun to his head before receiving one mother of a bitchslap, followed by the Batmobile chase - I'm guessing that Batman will be showing up near the film's end, once Joker has escaped/been released from Belle Reve (probably escaped, which explains why Batman is there!). Assuming that Harley spends most of the movie reclaiming herself and shaking off Joker's indoctrination, what with all of the setup for her origins and her prior relationship to Flagg, having Joker show up at the end to break her all over again would be the perfect cap to her relationship. It would also show off that it only takes the right words and his trademark violence for Joker to put Harleen right back into the cage he built for her as his #2, and that level of psychological monstrosity is what makes him the Clown Prince of Crime.