You can't, but you can delete all your Achievements and start over.
Get achievements...
The dead ringer in tf2 can be obtained by a timed drop, or by completing Spy Achievements
The Bonk atomic punch in tf2 can be obtained by a timed drop, or by completing Scout Achievements
You can get the gunslinger by completing Engineer-related achievements, or from a timed drop.
No, since your achievements are stored inside Steam Cloud, otherwise people could just edit the relevant file(s) to get all the achievements.
Its normally worth 1 scrap
you can receive them via a random drop about every 30min or get them via achievements. you can also trade them with other players
About 0.33 to 0.66
About 1 or 2 reclaimed
none unless u buy a key
Your TF2 may appear rainbowy because you have an item equipped on to one of your characters, such as the Pyrovision Goggles, which activates Pyrovision. To disable it, remove the item that activates pyrovision from your loadout.
A clean Backbiter's Billycock is worth 1.44 refined metal, according to backpack.tf