Every time you prestige you get another custom class.
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Probably not, but hay? Maybe.
You lose all the weapons you've achieved and you have to earn them back. You unlock new challenges, embles, titles, and more Custom classes. The highest rank Prestige you can get to is 10. After entering prestige mode, you are reset to level 1 in 1st prestige with a different emblem than the default one.
You need to be more specific on the "Diamond" there are a few emblems that look like Diamonds Here is a link to the list of emblems and how to get them: http://emotionalninja.net/modern-warfare-2-how-to-get-titles-and-emblems/776/2
get the perk scavenger u resuply from dead enemys
There are no cheats for Call of Duty: Modern Warfare 2 online multiplayer. There are glitches, but these are slowly being patched by Infinity Ward. There are also hacks, but these are more complicated to perform. Personally, I think hacks are pointless.