The are plenty of ways too get it early i.e. amazon
but also in those little private owned video game stores
As long as it a legit copy you will be able to play online if the servers are up
who knows if you will get reset on release like Halo Reach

as long as the copy is legit you can play whatever you want

GAME delivers games a day or two earlier, but it would be fine to play Multiplayer as long as you bought froma legitimate buyer. And if MS or IW or any of that non sense contacted you, you would say that you got it from GAME, which is a legtimate game selling market.