1. you can get a calling card (callsign) eventually and complete challenges for xp. also, you get credit again for the xp of unlocking the attatchments again. its good if you want to keep getting xp bonuses bc you know you dont want to use any other gun other than that one. but imo, the xp bonuses are rather insignificant compared to the xp it takes to level up to 55, so the only real reason to do it is if you're interested in completing the challenges and getting the calling cards.
2. dont think so
3. no idea
1. What's the point of prestiging guns?
Gun-Prestige Level 1 unlocks the ability to scratch your Clantag on that gun!
Gun-Prestige Level 2 unlocks the ability to put your own Emblem on that gun!
Similar to Black Ops 1!
2. Can you use prestige tokens to unlock a grenade launcher attachment?
May be yes! You can use your Prestige-Token to unlock any kind of gun, perk, attachment, ...
For all guns you´ve already leveled up, the attachments and colours does not need to be leveled up again!
You only need to level up yourself to unlock the weapon itself!
3. I heard people got free prestige tokens for pre-ordering, where's mine?
Watch out your primary-weapon, select it in your class-editor and push "triangle" to personalize it.
Press R1 (it think) and you´ll find "Prestige-Colour" - this should be the "free-prestige-token for pre-ordering...?!