There are actually a couple of people filling in at the SDC show while Nik and his wife Erendira are in New York. A friend, Blake, has been filling in for Nik on the wire, and was probably who you saw in the show, Gilligan. A woman named Hannah (I think) is filling in for Erendira with a silks act instead of the ring act.
And you're right, Nik wasn't happy about being tethered, and has even said it may hinder his walk. He fought it as best he could, but ABC said they would pull the plug if he didn't abide by that condition, so since they are putting about 4 million into this stunt, Nik has agreed to their wishes.