So, Melly's collar is back off. It wasn't actually a new collar; it was the same little elastic one she came with. I figured that the many tags (pet license, microchip, name & phone number) would be plenty jingly enough without a bell, but it turns out that Melly has a very smooth gait and is able to sneak about without the tags making noise. Next time I'm at the pet store, I'll find her a collar with a bell and we'll give this experiment another go.
In the meantime, Melly is very busy looking floofy and ridiculous.