The ballet shoes belong to the "fourth human" (never explicit but heavily implied to be the case) whose soul is in one of the jars like all the others. The ballet shoes manifest during photoshop flowey in one of the phases after flowey absorbed the jars. I just don't think it adds up. There is definitely something strange going on with plants and SOULs/determination and it's entirely possible alphys experimented on the grass but the "human died in the bush" theory seems far-fetched. Maybe alphys believed it could absorb determination or something to that effect so put it under protection and gave it to undyne in the ice cream because she believed her own strong personality might make it work?
Monsters generally can't handle determination because of how much weaker they are than humans, and undyne could well have generated some of it naturally trying to stop chara which would explain why it melts her. I personally prefer the idea that she comes back despite the disadvantage through sheer force of will but that is just a preference.
As far as anime goes this system is very similar to the human instrumentality project from evangelion.
Monsters generally can't handle determination because of how much weaker they are than humans, and undyne could well have generated some of it naturally trying to stop chara which would explain why it melts her. I personally prefer the idea that she comes back despite the disadvantage through sheer force of will but that is just a preference.
As far as anime goes this system is very similar to the human instrumentality project from evangelion.