With an early enough MacPro, there isn't hyperthreading.
as far as cores distribution, again, I believe in even as possible and using as many cores as possible. EG: 12 Kontakt instances same machine as PT, I am going to predicate distro on 2 cores for PT, even number seems ok. So, 3 instances of VEP, 4 Kontakt each. Set your VEP prefs to 2 threads per instance as you have four things to distro 8 cores to. if 24 kontakts, 8 ktkt by 3 VEP, 2 threads per instance. If you can get a maths where 1 thread per instance works, assuming you can starve PT to this extent... I think you follow my reasoning. Predicate the distro by what PT needs according to the load there.
But, I don't think that networking is your issue, because it
works for people. As per slave usage, just follow that, even distro and use up those cores.
I can leave my buffers at 128 until the cows come home, so long as I don't start relying on Cubase for a lot of FX. A whole lot of audio might get me there to sloweer performance as well, and I'll freeze that audio. Cubase is SLOW under OSX, it isn't really built right for it. This is why I rely on VEP on a network. I don't know PT, but tbh I don't envy you guys.
I THINK that in the case where problems have occurred in a setup, that corruption occurs. I do believe that when you're running on an overheated CPU and overheated RAM, this happens. In your case I would uninstall VE Pro, including preferences. The uninstaller provided in your VE Pro applications folder isn't going to trash prefs, so do that manually.