If multiple minions including Miracle Workers die at the same time on Geistwood, can Miracle Workers return themselves and/or another minion that's dying to your hand?

No, because Miracle Workers only return minions from the cemetery, and deathrites resolve while the minions are still in the realm. The Miracle Workers could return other minions that died earlier that turn and were in the cemetery.

If The Geistwood site itself and minions occupying the site are all destroyed at the same time, do the minions' genesis abilities still count as Deathrite abilities, or can the active player decide to destroy Geistwood before the deathrites resolve?

Since the destruction is simultaneous (e.g. Roots of Ygdrassil), all Deathrites will resolve before cards are moved to the cemetery.