Lower control arm shaft(some call it a pin) question

If the nut spins with tires in the air, then it will likely also spin with the weight on the tires.
I had that same problem, back in 1998, but I have forgotten how I solved it.
With rubber bushings, the pins are captured by the press-fit and so, yes, the LCA's have to be at ride height.
But not so with the loose-fit polys.

Oh I think I remember, now;
Those pins will stop spinning when they press/suck into, the K-member sockets.
Which means the LCA's don't actually need to be on the pins. With the LCAs out of the way, you can clamp the pins, up near the root, tighten the nut, then polish out the tool-marks so they don't eat your polys..