I believe the stranger will be Gandalf. Amazon has already admitted they're not following the timeline strictly. If it's not him it will be a Blue Wizard.

The best guesses for Halbrand are one of the Kings that later become the Nazgul, possibly Sauron (I don't believe it), or the Witch King of Angmar.

I just don't believe that Galadriel would be able to be duped that easily by the person she's sworn to hunt down.