From the sound of things, I'm betting she's a selfish little bitch that, since she can't get anything out of a fairy, she's trying to squeeze Momoka. I don't think she's been listening to you, frankly. You explained, fairly well I think, that Momoka already did everything she could have done, and yet missy's still saying 'you owe me.'
Matter of fact, I'm tempted to actually say words to that effect. Something like "Haven't you been listening? Or are you just trying to exploit this situation as much as you can."
Although your first listed approach of keeping blame on Dandelion is a good approach, too. Don't let her shift the blame off where it belongs.