I think Anna did put Elsa in the urn but it wasn’t because she thought Elsa was a monster. My guess is perhaps Elsa accidentally did something that she felt terrible guilt about so the rock trolls erased her memory at either Anna or Kristoff’s behest so that Elsa wouldn’t continue to torture herself with guilt and Elsa was put in the urn until things had died down a little but then it ended up in Rumple’s possession.
Also why is Elsa not asking where Kristoff is?
I was thinking either this or the Snow Queen tricked Anna into doing so and imprisoned Elsa by accident.