When she died for the first time, she was innocent, that was when she actually went to 'heaven'. We know this from her short story. So no, back then no one went to the Maw who wasn't supposed to. We also see Kael'thas and Uther as members of the other orders, so they didn't go to the Maw either.
When Sylvanas committed suicide in Icecrown, that's when she went to the Maw and she already deserved it then. While I think you could still argue it, I do think her refusing all of the chances the Val'kyr tried to offer her to be selfless and do something for her people (which one of the val'kyr was quite incredulous about) sealed the deal. that was also when she came into contact with the Jailer, which was only hinted at in her story, but she did see powers that made the Lich King seem small in comparison.
So in my opinion she's trying to set everyone free in the same way that Amon wanted to in Starcraft or the same way that Medivh agreed with Sargeras: by ending the world and sending everyone into oblivion (or the Void), because the world existing means that rules exist in order for it to continue. Those rules, that order is what is the prison for everyone and make everyone a pawn. But really, if you ask people, I bet they'd prefer existing to not existing, with only some nihilists disagreeing.