Following your logic Mind Control works against the Forsaken because they are marked as Humanoid. Could we not waste time with gameplay defiinitions?
If Sylvanas starts raising mindless undead at every chance she gets she will have even more targets on her back than before, so it’s not a preferable strategy.
Where did you even get the idea that Sylvanas is using Xal’athath to see what the Black Empire is up to? Last I checked, Sylvanas didn’t clairify why she wanted the Dagger, why it should be their key to victory, and the Dagger is pretty useless now since Xal’athath got her own body and bailed out.
The Void wants her gone as much as they want Turalyon and Vereesa gone, as much as they want gone anyone who is not their servants. And this hatred isn’t directed only at Death, the Void fights with pretty much every other force in the universe, be it the Naaru, the Legion, the Order of the Titans, and Life itself (remember Deathwing’s assault on the Life-Binder)? Again, they are not obsessed with Sylvanas, or Death specifically, and you cited that terribly-looking comic which shows the Void Lords trying to have Alleria kill Turalyon and Vereesa too.
Finally, Sylvanas calling Alleria an abomination for being a vessel for the Void does not mean ANYTHING when pretty much anyone who isn’t affiliated with the Void thinks that being a vessel of the Void makes you an abomination, that’s not peculiar to Sylvanas. Your evidence, if it can even be called so, because cryptic whispers from deceitful tricksters are not solid evidence, is flimsy at best and rather unconvincing.