The Orb is definitely a MacGuffin.
If you really want to define MacGuffin, go all the way back to the guy that coined the term. Alfred Hitchcock defined it as "the thing the characters are after" and " the thing the characters on the screen worry about but the audience don't care".
The microfilm in 39 Steps and North By Northwest, the letters of transit in Casablanca, the diamonds in Reservoir Dogs, the briefcase in Pulp Fiction, Rabbit's Foot in Mission Impossible III, whatever Indiana Jones is after (ark, stone, goblet, skull), etc. In Marvel's case, the Cube, the Orb, the Aether and whatever Thing they throw next are MacGuffins.
It's what characters are after or fight over. The point is you could replace it with pretty much anything and it wouldn't matter. The Orb could be The Rectangle and it could turn people inside out or create a black hole or reunite the cast of Friends, it doesn't matter. The important thing is both the good guys and the bad guys want it. End story.
To answer your ending question, a MacGuffin is NOT a MacGuffin when it doesn't guide the plot. Lightsabers in Star Wars? Not MacGuffins. The necklace in Titanic? Not really a MacGuffin as it doesn't guide the good guys' actions (their love does), just the bad guy's. The rug in Big Lebowski? Starts off as one but ends up as an excuse to kick the plot off rather than guide it.