You unlock it at level 36.
By either completing all the challenges available in the Barracks section, or by hacking.
You cant.
No.
you have to reach level 50
Get to Prestige 9.
you can unlock it with you're first killstreak award unlock
You unlock it by getting to 10th Prestige.
you cannot unlock them you have to buy them on PSN or LIVE Marketplace
Sorry there is no alien invasion
Level 38
Level 48