Hey how's it going ?
1) if it is stamped 223 on the barrel I would only use 223 ammo .
2) yes but you could contact DPMS and ask if you can fire 5.56 out of there heavy bull barrels
3) I would also Recommend changing out the FCG . The problem is most likely the diconect or the hammer . A FCG is about $40
4) CLP all the way baby