Whilst it was wrong that Rumple switched the daggers, at the same time I’m glad he did it otherwise he would be forced to lead Belle to a very dangerous woman with no guarantee that Belle would walk away unscathed.
Except for the fact that it puts Belle in danger anyway because she can’t use the dagger to defend herself. Belle’s not a fighter.
I remember I theorized if Belle used the dagger to defend herself and found that she couldn’t because it’s a fake, she’s basically on her own and defenseless. In this case, I think she’s probably lucky because Rumple is with her but if he weren’t, if she were going solo to confront the Snow Queen, there’s no helping her. She would be on her own with a fake dagger that can’t summon the dark one to save her.
I mean, look at Belle’s body language and the freaked out tone in her voice, she’s horrified. How much more embarrassed and scared would she be if she knew she had a fake dagger that she can’t use?