maybe...just maybe...

the witch was Betty...she went crazy too after the mushroom war and found ecstasy on draining all those aura filled with emotions that comes with your favorite things (a subconscious longing for the affection and happiness that she felt with Simon), kinda like...yeah...Ecstasy or other party drugs...

So I infer that she holds a trade every now and then for magic potions and other tools- one important object that a person holds near and dear for a powerful magical object; of course, the bitch Ash just loves power and saw her items of being in his interest. So he got Marceline's bear ('coz a bitch like him isn't capable of that kind of deep affection) and sold it to the witch.

The witch got a hand of the bear and she sensed that ecstasy that cannot be compared from the others she drained-It was the warm and affectionate aura of her dear love Simon; even though she remembered him, she was too insane (and "emotionally high") to think ethically and consider that this might be the most precious thing of a child, and it would brake that child's heart if that was lost. But things are already done; she kept her best "emotional drug" and gave Ash the best magic tool she had....



And I also bet that the flower was of significance of her and Simon (if my assumption of the witch being Betty is correct)