After Lisanna died in the accident, it is understandable when one sees Elfman's apprehension/fear that he will lose control over his full body takeover again- the reason why Lisanna apparently dies.
I understand Elfman not being able to use magic, but Mirajane? She was an S-class mage. Why does she end up waiting tables at Fairytail? Did her magical prowess diminish after the incident?