Just reread the chapter. That was a very emotional and sort of depressing chapter wasn't it? I mean I loved it, but it was pretty deep and dark.
I have the feeling that the box is not just her feelings, or her heart, but also her memories that are being unlocked.
I think that Shou will also have a lot of character growth and I don't think that her relationship with either guy will be soon cause both these men need a lot of growth to be good in a truerelationship.
Shou and Ren both have problems and she is linked to both of them as their "savior".
And I think that a lot of mangaka's that have something this in depth don't want some simply solved solution, the writer is going for something deeper than just romance.
Anyway with Ren being nice to other people and her thinking he's the "king of the night" I think she might not want to fall in love with him not only cause she doesn't want to reveal her feelings but also since
she doesn't want to get hurt. It's not just the memory of another love hurting her, it's her thinking it will be the same thing with her devoted to someone who is not exclusive in his affections to her but
nice to everyone - cause it can feel like a form of cheating or betrayal. She is already jealous with that and doesn't want to feel that way again. So she is not going to let herself be hurt.
Ren was seeing that other girl the same way he used to see Kyouko definitely, as someone who needed to be protected, not as a woman on equal footing to him.
I found it curious that Ren kept asking her if she was angry when she really wasn't feeling anger but jealousy and he kept pushing her and asking her like he was trying to get her to admit something and she would not.
I don't think the locks are just emotions either but memories which could be a lot more convoluted.