I like this theory, I like it a lot. I choose to believe that they have turned into trees in the Shire, though, because I agree with Treebeard that the Shire does seem exactly like the kind of place they would enjoy.

Fëanor is the world's biggest failboat, how he lives as long as he does is a terrible mystery to me. ...I admit to not remembering Haleth, it has been a long time since I read the Silmarillion and I'm only up to Chapter Fourteen in the Rewrite. (BUT I'M ABOUT TO GET TO FINGON AND MAEDHROS ZOMG.)