Fair point on the warlocks, hadn't really considered the consequences of that sorta "casual" soulfuckery.Il'gynoth returning to Ny'alotha would be another of the 1,001 exceptions I mentioned at the top - although it's also possible that the Ny'alotha we see in BfA, and the one where we encounter him "reborn" is itself an extrusion of or part of the Void itself. Which would make sense to me, since N'Zoth like all the other Old Gods are also born of the Void, and likely return to it on death, explaining in part why portions of them are still around and seemingly "alive" to some degree.
Helheim's another example of an exception where a higher power, in this case Helya, can mark souls for different destinations and disrupt or interfere with the more natural order. Odyn does the same with his Valarjar and the Halls of Valor - he uses his own personal Val'kyr to mark the souls of brave Vrykul (and other beings) and brings them to his own private "afterlife," just like Helya does with the souls she manages to snag. Both Helya and Odyn put themselves into the mechanism that would otherwise deliver those mortal souls to the Shadowlands.
So yes, death is still a clusterfuck, as you put it. There are some rules in place, but those rules can and often are broken all the time by beings powerful enough to do so. Hell, even mortal Warlocks can snare and even destroy souls for their own use - death in the Warcraft universe isn't sacrosanct, and successful delivery of any soul to an afterlife scenario is most definitely not guaranteed.