Need help in multiple areas for a magic tools mod

OK so I'm working on a mod that has magic tools in it. For many of the magic abilities I don't even know where to start.

I'll start with the easiest, the Bow.

I want the bow to start out as just the handle, then when you switch to it the bow extends out and puts itself together.

With this one my only problem is I don't know how to change the texture upon the user switching to the bow.

Next is the pick, I want the pick to be run off a charge, and as the charge goes down the pick has less effectiveness (I already know how to do this). But then when you are within 10 blocks of a magic block the pick will start charging back up. And If fully charged you can hold right-click for a couple seconds and the pick will dis-charge entirely and destroy all stone/netherrack blocks within a 25 block radius.

Lastly is the sword. For this one I want it so when its fully charged, you can hold right click for a couple seconds, then the sword will hit and you will hear a click, then if you right click again you will hear an explosion and it will kill all hostile mobs within a 100 block radius.

All I need help on is I don't know how to:

A) make the bow change textures upon being switched to

B) make the pick destroy all stone/netherrack blocks within a 25 block radius

C) make the sword kill all hostile mobs within a 100 block radius.