How–door—how?!?!
Oh there’s that magical door to Arendelle again. Hmmm. My own guess is that the apprentice gets freed from that hat (as do the fairies, and perhaps the “queens of darkness”). The apprentice then presents the portal to Elsa, Anna, and Kristoff, allowing them to return home.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy