Thanks for the chapter!
I am guessing with the memories that demon is remembering, that when they don't have mana for attacks they use their bodies in the Astral(the realm where summons come from), so it's not that they need the mana stones, but not using them brings harm to their real bodies and the demon is just insane or unaware.
Could be since the main relation we saw was when her eye was obliterated and the vision we saw had a similar instance happen to her eye... Though for some reason got pierced from the back of her skull through her eye in front.
It does make one wonder what's the limit to their possible insanity since she was trapped in that demon eating tree, unable to move or use magic for all eternity.
And maybe that vision is her actual body like you said so her current form is either an illusion made with magic or something else entirely.
Syplhy is 'power saving' so the demon would be... Full power output? Excessive consumption?
The theory earlier can't seem to explain how it could relate to her outward appearance yet and what possible harm it could bring aside from exhaustion I assume but it's a great theory nonetheless