What grip should I use for overhead clears so I can do backhand drops, smashes and clears? And btw how should my arm move for these techniques? For forehand shots like drops, smashes and clears, how should I move my arm? I usually take a full swing by bending my arm behind me and snapping my wrist. Thanks

The grips are just different for backhands and forehands, all forehands use the same grip, backhands is a bit different in some cases, but a standard backhand grip will do for most shots. Pronation and Supination of the Forearm is what you are looking for. Swings and Snapping are situational, maybe sometimes you just wanna snap when you are doing a net kill etc.