Is the aluminum threaded part of the NSS still all there, or did it break off with the nut? If it's broken, you may be out of luck. The nut is steel and the shaft of the switch aluminum, so if the nut is messed up it may be because it's full of that aluminum. You might still be able to get the switch off pretty much intact, though you probably can't save it if there's nothing left to screw the nut on to.
Whatever you do, do not try to expand the NSS shaft by prying it apart at the slits. It will break instantly.
It can be hard to get off if the aluminum hollow shaft of the NSS has seized to the steel shaft that goes through it. What you can try is first, to get something behind the body of the switch, and pry it outward a little. Not too hard: it's plastic. All you want is to move it out as far as it goes and have something to hold it there. A thin wooden wedge would be fine. Get plenty of penetrant on the shaft. Then take a small hammer, and tap the end of the steel shaft. There's a very little bit of end play in the shaft, and if you can tap it inward, it should break the corrosion bond. If it does seem to move, don't pry the switch out immediately, but try pushing the NSS back in toward the transmission, in hope of getting it to move back and forth a little on the shaft, and to carry some of the penetrant in. If you can move it back and forth a couple of times, it should slide off fairly easily eventually.