[@Caits]
If it got broken then yes the mage would be the source of magic to regen it. Of course once it gets broken to a certain point, and the mage runs out of energy it would simply stop regenerating and just be broken as the mage would be unable to muster energy to move.
Indestructible might just be because of materials used to make it. How about just make it difficult to break?
Like the blade is made of mithril (assuming Fairy Tail has mithril XD) and the handle made of mahogany (unsure how heavy that would be) and it's magically infused during the process of creation to make it difficult, but not impossible to break.
EDIT: Or maybe her magic can enhance her scythes durability as well as her own? Sense she will her magic channels her magic into enhanced strength and speed, it could in theory be channeled into a weapon to strengthen it slightly and maybe even strengthen her own durability.
Also what about this for Manami?
Water Saint: A lost magic that has some holy characteristics to it. Her element water mainly consists of defensive and healing spells, but still has plenty of powerful attacking options.