Aluminums are good but will bend. I shoot carbon, currently using TenPoint pro elites but will swith to Black Eagle Executioners in the future. No, accuracy does not decline after a certain number of shots with quality arrows.
Mechanicals and fixed blade may or may not fly like a field point. Broadhead manufacturers have done a great job of marketing that their heads fly like field points. Also, do not assume that the practice head or blades will fly like the real broadhead. Every bow is different, you have to shoot them to know for sure. To assume your broadheads have the same POI as the field points is pure laziness. To go in the woods and aim that untested arrow and head at an animal is unethical.