The 300BLK can be a bit tricky to initially work out (similar to AR pistols) with common problems being jams, cycling trouble. It should be smooth once you get the correct parts with quality mags and ammo. Here are a few recommendations. If you make any changes to a factory built upper (removal of gas block or handguard change), make sure the gas block is correctly replaced to completely cover the gas hole. Also ensure the lower receiver's buffer tube is not over-rotated, so that when the upper and lower are put together, the upper doesn't hang up on the buffer tube (instead of seating properly into the lower). Over-rotated buffer tube can cause the carrier to ride up and bind the action.
1: We definitely recommend a standard 3oz buffer if not cycling.
2: The spring really depends. Carbine springs are not created equal, so you don't necessarily need a reduced power spring, but it is always a good place to start if you are having issues with a 300blk. I offer lightweight option AR15 spring here:
3: H2 buffer is definitely going to be a no go, at least with subsonic ammo.
4: 300blk is such a low pressure round, I recommend not even bothering changing the Adams Arms gas positions when switching from subs to supers and just keep it on full gas.
Adams has changed from standard 1:12 twist to 1:8. With the 1:12, anything bigger than 190gr will keyhole slightly. They changed to 1:8 so that this will not be an issue.