Both disasters have the same root cause - these are emulsions, and emulsions hate two things: freezing, and being worked while cold.
Ganache: fridge, 1-2 weeks, cling film pressed onto the surface. Never the freezer. Ice crystals rupture the emulsion and you get exactly that grainy oily split, very hard to bring back. To reuse, warm in 5-10 second microwave bursts stirring between each. Slightly separated? A splash of warm cream whisked in usually pulls it back together.
Buttercream: your curdled soup was temperature, not spoilage. Cold butter beaten hard breaks into fat lumps floating in liquid - the classic curdle. Bring it FULLY to room temp first (genuinely soft, couple of hours out), then whip. Comes back smooth almost every time. A curdled batch can even be rescued mid-whip - keep beating while warming the outside of the bowl for a few seconds with a warm cloth, it snaps back together.
Keeping times: butter-based creams 2-3 days surface-wrapped, ganache-based as above. Honest production advice though? Make frostings to the week's need. They're quick to make and always best fresh.
