Insofar as we know there are two methods to get tossed into the Maw: 1.) failed to be redeemed in Revendreth, and 2.) be a threat to the Warcraft metacosm, specifically to the Shadowlands themselves. Kael'thas is confirmed to be in Revendreth, and Garrosh would also likely be consigned to that location. As of the time of Edge of Night I agree Sylvanas had not yet done anything that would take her straight to the Maw, making it very likely that her visit there was arranged beforehand as a result of external tampering and not where she was actually supposed to go. Devos and Uther weren't concerned that Uther could escape the Maw, per se; but more that the Arbiter might not consign him there directly (instead sending him to Revendreth or somewhere else entirely) thereby denying Uther his justice (or vengeance). Devos also considered Arthas' existence an existential threat to Bastion, which none of the other Kyrians believed.
Before the machine of Death was broken the Maw would've been a very rare final destination for a given soul. Revendreth has confirmed world-destroyers in it, after all; so the vast majority of souls would at least be sent there before falling into the Maw. I think Gul'dan might be a likely candidate for the Maw himself, considered that in his life he not only destroyed worlds but also disturbed the very fabric of the metacosm by messing with the spiritual realm of the Elements and utilizing the Cipher of Damnation. He could very well be seen as the kind of threat that makes a soul worthy of the Maw directly. Demons can't go the Shadowlands in any case, but Kil'jaeden and the rest of the Legion leaders are probably similar in that regard.