Can I get info GrandHunt?

How can I get information about the great hunt or other current events in the game?

no there is no endpoint for this information unfortunately

How does WowHead get event data on the front page or WeakAura plugins add timers?

Looking into this myself as a long time developer, first time Blizzard API fool arounder.

wowhead uses there game client to pull data and user uploads and then they have the basic server timings for things thats how they do it