Eventually it behaves itself and they have not broken its glowing property in the course of the attempt!
Bella gets underway on a version that gives a bit more distance (Fëanáro may have to share a room with people he doesn't want to hug him; people fleeing from orcs presumably have no such consideration and may be less polite about repelling orcs into walls than the subtle give worked into the ring's magic.)
Here, would somebody visiting from the Outer Lands like to take this prototype and whenever (hopefully not too soon) they have cause to see it work let her know the results?
That might be it; Bella doesn't know much about how the anti-decay thing works but has never thought of growing to adulthood as decay.
Valinor is weird. Bella's glad it doesn't have to be all or nothing anymore.
It's not that it hasn't occurred to her, but she'd cope less well with eternal night than with eternal day and also has had enough of living in places with things that want to kill her all the time.
And Bella goes back to everything else. There's plenty of it. She's getting pretty good at working at a respectable speed without the necklace; she just has to pay attention to herself (and wear a watch, and set timers). She's good at paying attention to herself.
"Learning better self-monitoring strategies to keep all your hours in a row without the necklace?" Bella asks.