I managed to unlock my first witch (witch of thunder) and wanted to know whether the fragments could be collected again. And I guess the second part to that is, are any of the witches worth collecting for F2Ps?
i personally have not tested getting multiple witches (for the purpose of overdriving). On the subject of collecting, their viability tends to fall off to other 5*+ divers or even some 4* ones.
No, however one of the items in the exchange shop does raise the over drive for any unit. Although I am saving up for king enma first (5 star fire and dark).