Totally late to the MW2 party I know, but picked it up the other week and been having a blast.
 
One quick question - is there some kind of way to use my online profile offline on all the accounts playing? I messed around with the settings and could only get my online profile to 1 player, what about the other 3? Are they relegated only to the sparsely used offline accounts? I don't want to have 3 mates rock up for me to be the only one rocking some sweet weapon. After a bit of googling, I found a 'hack' to get offline profiles to have all the unlocks - which I suppose is the fallback plan if there's no way to do the first thing. Anyone done this and not have it screw up on their game data?

I have absolutely no idea what you're asking. 
 
If you're playing spit-screen on 360 everything is unlocked.