The Xbox has nothing to do with the ranking on Call of Duty, thats the online game developers of COD (Infinity Ward) error.
No If you play campaign online you can get exp.
Yes, you earn XP for playing online, leveling up in rank and unlocking weapons and perks.
no
you rank up to level 65
no, your online rank will only be deleted if you delete your xbox live/psn account.
70
you cannot get a rank offline. only on the online feature. if you play offline it is either in story mode or with a friend.
In Campaign you can easily find it on dead enemies in Online Multiplayer I don't remember what rank but you have to unlock the FG42 at a certain rank
This is called deranking and usually resets you to level 1. Unfortunately there is nothing you can do to fix it, you will have to work your way back up.
Yes and I believe it is one of the best online games in the series......bnfsmf - leaderboard rank 198
keep playing the campaign or if you want a rank online then play online
you go to the grand cannon