I had been thinking the voyage would take long enough that he'd turn. Maybe something unexpected provokes him and "makes" him turn?
While I agree that the show isn't required to be set in London, it seems most likely it will be set primarily there again. The Murray house feels like the center of this universe to me.
The only reason anyone has to go to the US right now would be to find Ethan. Maybe by the end of season 3 they may all become fugitives and escape there, but right now I don't see why Frankenstein or Malcolm or Vanessa would go other than to help Ethan.
I'm curious how they even find out Ethan turned himself over and was sent back to the US in the first place. The man that was extradited is Mr Talbot, and they know him as Mr Chandler. Maybe there's a sketch? I'm also wondering if another person from his father's clan comes looking for him who turns out to be another werewolf sibling, or becomes an ally to them.