I thought that Rumpel had Tiana hold the dagger because he was trying to figure out of she was the guardian. It is interesting that Gothel is looking for the guardian as well, but why? We don’t really know enough about her character to even guess at it.
I think the implication of one of Tremaine’s daughters is the guardian, is that it will be Drizella and not Anastasia. I don’t think Anastasia is important except as a plot device.