To add a little more to the Time Travel or Traveled theory:

In Cold Days, when Harry and DR entered to first room under the island: Harry realized that this room was the place where his body was being maintained during GS.  DR looked at Harry and repeated 'Memories-Reflection'several times... I think that there was more significance than just 'an internal monologue' as Harry thought.

DR could be hinting to Harry that maybe Harry had been in that room before (besides GS)... maybe as Merlin? ... Or in Merlin's mantle?  Maybe the title 'Warren' is a bit more than a title... It could be a mantle...

Harry had been told by Odin and the Gatekeeper that his near-death or death-resurrection experience had changed him... Harry even felt that he was changed... changed to what?