Cheating is cheating whether it's online or not. There is no difference. You still effect other especially if anything u do in say spec ops goes on a leader board cause u scored a ton of points you still effected others.
No it's not. Cheating in multiplayer gives you advantages over actual players, not NPCs. The other difference is, if you're discovered, you'll be reported and banned from the game because online/multiplayer cheating is forbidden by the game's EULA.