Lower control arm pivot shaft woes

I always use poly so it doesn't matter to me what position the LCA is in when the nut is tightened. (Don't twist the rubber after one tightens the nut and the arm position is changed.. Same with the UCA).
That being said, I've also never had a problem tightening the nuts.
That has a shoulder on it, right? It draws up as the nut is tightened.
You said it's got oil on it. I bet if you clean it. and help it get in the hole from the rear as you draw it in, you problem will go away.