In episode 6 of 2wei (not sure where that is in the manga), Irisviel returns to Fuyuki and meets Kuro. She delivers exposition to Miyu, Kuro, and Illya in the Edelfelt mansion's unreasonably large bath. In particular, she informs them that the Holy Grail War is nevermore to be; Kuro does not take this news well, and starts venting mana (or something).
Anyway, despite Illya's (ahem) mana transfers, Kuro seems to be fading away due to a lack of mana... until she has an emotional moment and admits to herself that she wants to live.
What is the mechanistic explanation here for why Kuro didn't continue to deplete her mana supplies until she vanished altogether?