She memorizes the rock from every angle. Then she changes into a jacket with pockets, pocket the rock, and changes out of the jacket.
The rock sure is deleted.
Can she get the jacket with the rock back—yes. Same rock, when she inspects it.
She repockets the rock. Can she disappear the jacket and don a different jacket with the rock in its pocket? No. Can she move the rock to a different pocket on the same jacket? No. Can she just vanish the rock without changing her jacket and get it back the same, by "changing" into versions with and without the rock? Yes.
She puts the rock back on the table and tries to summon the jacket with the rock again... no.
She tries to summon the jacket with the rock again, picturing the exact rock and exact jacket—that outfit she was wearing a few moments ago—that works.
She memorizes her pile of rocks, turns around, scrambles them around, and pockets one without looking. She unjackets, rejackets, takes out her pocket rock, duplicates the rock, and takes out the duplicate. She turns around and checks her collection against the two rocks she's holding. Did it successfully return and copy the rock that she took from the pile?
Yes.
So Dressing Room acts as a pocket dimension, in a way. You can get back out objects which you put in, even if you don't know exactly what you put in. You can duplicate objects with more effort. She'll need something more exotic to know if the objects stored still meaningfully exist in a way when they're deexisted.
She examines the coin in her reward pouch. imagines the outfit of a rich merchant from historical China, with gaudy flowing robes and, importantly a small purse of coin at his hip.
It works.
She tries to think, this is my purse now, I own it, changes back to her previous outfit, and tries to put on a jacket with her coin purse in the left pocket; no dice—no, that wouldn't work since she can't transfer pocket contents between outfits. She changes back into merchant wear, removes the coin purse to the desk, then changes back into her normal clothes, puts the purse into a jacket pocket, and rejackets. The purse stores and comes back fine. Maybe she'll be able to do the first thing with more practice. She duplicates the purse a few times and puts copies into pocket storage on a few different outfits. Can she also just don the coin purse on arbitrary outfits as an accessory itself...?
Once she has a handle on the right frame of mind, yes.