Of course any individual audience member can interpret things as s/he chooses. But that doesn't mean that the reading is necessarily supported by the text, or if it is, that it's the best choice to keep the franchise dramatically viable. It doesn't help that the text itself can't seem to make up its mind in this case, where the entire storyline seems to be about recovering from loss and moving on, which works against the notion of unique love. I'm responding to your supposition that the character of Martha is intentionally introduced as a whipping girl, a "not Rose." You may be right, but I think that's a dreadful dramatic choice to make for an ongoing drama, making everything that comes next into an extended, unpleasant epilogue.

Anyway, I seem to have ruffled your feathers by disagreeing, so I'll just leave it at that. Thanks for engaging in an interesting discussion.