I expect it is the demon that actually feeds, not the "human" side. So the demon has to be active
Also, Martin and Susan were born human. Whamps are born Whamps. Nothing to go back to.
The feeding would have to happen before the demon was set to sleep.
And because whamps are born with their demon, it has to remain for them to survive. The hunger demon is a separate entity (soulgaze with Thomas in Blood Rites), more like a symbiot for the human part of the whamp and not like the ramps who become fully vampire. But I hope it can be inactivated like it basically did in Inari or any other Whampire who felt true love at their first time.
I believe it is like this: remove the demon and the whampire dies. Inactivate it and the whamp survives. They become human again. No more feeding needed.
Thomas would probably need a final feeding to heal completely. But after that the demon might just continue. Lara said something like once it starts to feed off it's host there is no going back, didn't she? I am not sure. She did in my head, lol. I will look it up later.
This is what made me think of inactivating the demon instead of removing it.
Edit:
Here it is: Peace Talks, chapter 27. Harry asking Lara, if feeding on Justine could help Thomas. “No,” Lara said, her eyes intent on his face. “By the time a Hunger turns on one of us, it’s mad, uncontrollable, insatiable. Even if we could redirect the Hunger, it would kill her and the child, and he’d die anyway.”
I understand this as once the Hunger feeds from it's host, this process can not be stopped.
