You don't need scripts for this. Just set silver arrows and wooden arrows as the "item override" in the subscreen for two different slots, and you'll be able to have both. At least, I believe it can work that way. I know DarkMatt did this with silver arrows also costing magic in his Fun in the Sun II.

Base Speed for Short Bow is 100. Scripted weapon speed is measured in 1/100ths of pixels per frame.
Piercing is an option in the item editor for silver arrows, though yo'll have to manually keep it alive if you go with scripts.
Sparkle trail you just set the sparkle sprite in the item editor and it'll enable it. If you wish to do this via scripts, just create LW_SPARKLE at the arrow's locations every few frames.