I think the most interesting scene in part 6 to me when we were playing through was the scene where it was proclaimed to not be “Battler’s turn”. When we were playing through, I thought it initially indicated that Battler, on the gameboard, was aware of the other gameboards. This was also (I thought) reinforced by a quote from the airport scene where it seemed Battler was aware of something Maria had said in a scene when he was not present (in editing I’ve realised this was just just a variation of “trick or treat” that I’d not heard before). Post editing I think that it’s more likely that it’s actually the narrator of the gameboard trying to push Battler to actually pay attention to everything that is happening, though I’m still not too sure of why.
As for the other things we addressed in the episode, which I clued in to your interest for by means of magic, I think that the story is designed such that Rokkenjima is actually isolated. I believe the storm is meant to be flimsy enough of a restriction such that Ryukishi can still pose the question of others arriving on the island, but I think this is there just to establish a possibility to misdirect.
As for the letter question which you also definitely did not post, I think this is largely for the culprit to maintain the mystique of the witch. Whilst the witch is seen by Kyrie, it’s possible this was unintended, thus why she is seen as dressed differently to the portrait. I could go in to more theories on this here if you want but it’s getting late and this is my main thought