As for the necklace, I think its a flashback. Glinda said previously that she gave Zelena the necklace to store her magic. So if the sneak peek with DG is when Glinda and Zelena just became friends maybe the necklace is clear because Zelena hasn’t stored her magic in it yet. Her magic always seems to be green ie. with the cursed potion and when she disappears and what not.
Yes it’s Oz of the past, we’re just not sure when since Zelena isn’t green
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"He was a lot of things to me" "The only conclusion was love"