What is the purpose of the cursed art gallery?

Spoilers!! Be warned

I've ended the game Ib and got all the endings, but seems that the game doesn't clarify some things within the story. The main question that I had was, what is the purpose of Guerterna's cursed art gallery? And, why was Ib called to come to this place?

I thought it could be Mary's doing, as like her diary said, she wanted to go outside and had a real life, but needed to take the place of another person. But, in A Painting's Demise ending, we learnt that even Mary isn't allowed to leave. So, I'm out of options.

If someone could clarify, I'll be grateful

P.S. Also, why Ib was called a "thief" when she got her first key? As I read in vgperson's translation notes, the original word was "kaese" (give it back), but still it doesn't make much sense to me. Is it supposed to say to Ib to return the key? Or the rose? Or what would be?