I am going to type this taking Sylvanas’s actions into the best conceivable light (she’s right and the Jailer is exerting undue influence on Azeroth and needs to be stopped). If Sylvanas is trying to do good for the world of Azeroth, then she is by miles the least competent leader it’s ever been cursed with.
If something is happening in the afterlife to those that die, every single living thing on Azeroth is going to want to figure out what it is and how to fix it. This is a planet-uniting threat, provided she has the means to prove it (and given the powerful spiritual connections available on both sides of the faction divide, this shouldn’t have been difficult). That she decided going rogue was a better plan than facing the threat with a fully united Azeroth (you know, the kind of force that gets things done when big bads come knocking) behind her speaks of her lack of leadership ability, and should honestly call into question her very sanity.
This is, of course, assuming that literally everything about Sylvanas is true from the loyalist perspective. Which calls several of her actions into severe question. Why slaughter NE when you’ll need their aid in your efforts against the Jailer, especially if their deaths empower this entity? Why violate your very own oath to your Forsaken people and re-raise them as mindless undead? Why push things to such an extent to have the world almost crack under the pressure of mortal threats, let alone the afterlife that awaits?
Her opening the Shadowlands isn’t her trying to leave us breadcrumbs, it’s her setting a trap and praying we’re not good enough to get out of it.