So all you can use the mirror shield is to deflect back SOME enemy shots. Just from memory, I think you honestly can only deflect back wizzrobe shots and zora shots, and that's it. I know you can't deflect arrows, rocks, boomerang, swords, or fire back at enemies.
By default, yes.
However... projectile flags.
Rework the numbers in the item editor to decide what projectiles the shield blocks and reflects. Only arrows / boomerangs / fire can't be reflected at the moment. The "Defenses" tabs in the enemy editor already have entries for how they react to reflected sword beams and rocks.
But, you can block fire with the mirror shield, which I don't believe is possible with the regular magic shield.
Same as above: change the Magic Shield block flags from 191 to 255 to make it block fire.
ah, i see. but why are there magic prisms and magic mirrors in the game then?
Those are combo functions, not item functions. In other words, they're in-game objects that are just sitting around, not something for a player or enemy to "use" (although they can both interact with them).
Their default functionality is related to magic attacks, however, if you go to Quest Rules -> Combos, you'll find checkboxes for "Magic Mirror/Prism Combos Reflect Sword Beams" and "Magic Mirrors Reflect Whistle Whirlwinds" if you want to expand their functionality.