I have been using the Arradondo slide for about 20 years. Great product. BUT, do not use with any type of stick powder. It will damage the charge bar. For very fine flake and ball powders, it works really well. A lot less sifting through of powder.
I use small rifle primers in just about everything I load. .380, 9, 40, 38sc, etc. You need/should run rifle primers at major pf. That way you get less primer flow and flattening.
Not legal. IMO, you are using the other arm as a support. Hold the free hand tight to your chest. That way it doesn't flop around like a pendulum. You can't support the other arm in any way.
I have been running AFTEC in all my 1911/2011 pistols for 20 years. I have never had to change springs or broken one yet. Knock on wood. So, what's that tell ya.
4.8-5.0 should put you in the right ballpark. I use 5.0 with a 124 FMJ at OAL of 1.165. I load 9 major at that length, so I just load my minor stuff at that length as well. Runs 100% and makes around 130 PF.