Any question answered...

ftfy

Splines yes, Halo splines maybe not. I don't think the Shimano HG spline on which it is based is well designed for reversing loads, and backlash not only wears the splines but also tends to unwind the lock ring.

The obvious solution seems to a trapezoidal spline retained with a threaded lockring; any system with square splines is never going handle reversing loads, no matter how carefully designed or manufactured. Anything is better than crumbly threads in aluminum though.