The Midsouth Varmint Nightmare bullets will do just fine in your AR. If you don't have them already, you better act fast as the only ones in stock are my favorite 55gr Hollow Point and they will not last long.
http://www.midsouthshooterssupply.com/department.asp?dept=RELOADING&dept2=JACKETED+RIFLE &dept3=.224+DIA&dept4=TRADITIONAL&vendors=MIDSO UTH
in a 16" 1:9 barrel. It's a faster twist than spec'd but with a shorter barrel I was thinking muzzle velocity and hence rotational forces would be lighter with shorter barrel.
Note sure what you are getting at here, sorry.
The twist rate listed in the loading manuals is of little importance, I ignore it.
Your barrel length will affect velocity and that's about it. If your load data used a 24 inch barrel then expect the same load in your 16 inch barrel to be about 200fps less.
Your 1-9 twist is a good compromise between the 1-7/1-12 twists out there. You will be fine with bullets from 50 to 70gr. You may even have good luck with lighter or even heavier, but to start out I would go with the 53-55gr.
From Hodgdon on IMR 4320.
Short granulation, easy metering, and perfect for the 223 Remington, 22-250 Remington, 250 Savage, and other medium burn rate cartridges. Long a top choice for the vintage 300 Savage cartridge as well.
You will be fine with this powder, just expect your charges to fill the case and eventually compressed. Don't get nervous about "compression" it is perfectly safe and preferred by many.
Note the "C" in Hodgdon's data after the max charge if 4320 with 55gr Speer SP. The "C" means compressed and is expected and it's going to happen, go with it you will be fine. Just add the powder slow or with a long drop tube so as not to spill it over the top.