I've been using the Nx without the head tracker and the only problem is now the straight headphones sound really strange and unnatural. And that's not a real problem. I like the Nx.

NX is useful, but the head tracker gimmick is pointless. Unless perhaps you really want to know what your mix will sound like to someone sitting facing away from the speakers or moving around. It can also be pretty heavy on cpu.

It does really need to be used with Sonarworks or some other method of flattening the headphone's frequency response though. A task which would be much easier if headphone manufacturers supplied frequency plots of their headphones so a reverse eq curve would be fairly straightforward to set up to remove at least the bigger mountains and valleys in the respose plot.

Monitor, speaker and microphone manufacturers supply frequency plots for even inexpensive products like the SM58 or 57. That few headphone manufacturers do may just say something about their headphones I suppose.

I just built a test project using Melda's Noise Generator and Autopan(so the sound would move around) and putting NX and Sonarworks plugins in different orders.  The order of the two plugins didn't matter, the audio matched exactly (inverted the phase on one nulled out the other). 
 
Not sure if with very quiet sources (or loud/clipping) there can be issues with the EQ curves causing frequencies to completely vanish but loud (but not too loud) pink noise being autopanned  (or not, tried it both ways) didn't show any difference.  This is with NX with it's default settings (no head tracker) and Sonarworks with the reference ATH-50X profile set to Flat.
 
After this test, I'm no longer thinking it matters what order you put the two plugins in.

I agree - logically it shouldn't matter which plugin you use first.  NX-VR addresses panning, Sonarworks addresses EQ.