The spindle nut is held in place by 3 set screws that go in to the collar and lock in wedges that keep it in place. If I recall, once the set screws were loosened, mine was pretty easy to loosen. There should be a decent size gap between that spindle collar nut and the base plate on the top of the head. The socket that comes with the mill makes it easier to remove, but I bet you can remove it without it...
Also, in case you weren't aware, there are exploded assembly drawings on Novakon's website
https://www.novakon.net/products/pulsar