Spirit Laura - is the One
I believe the "bubble Laura" created by the Giant is a good spirit who was sent to counter evil spirit Bob. The spirit "Laura" would be like Bob - she possesses or lives with others, but does not have a true physical form on earth. I do not believe the "bubble Laura" was the human Laura Palmer or the frogbug that crawled in the girl's mouth. "bubble Laura" has been a spirit on Earth since the Giant sent her - just like Bob has since mother/experiment sent him.
This explains the early phrases by the arm: "she's my cousin" (because they are spirit cousins) and by "Laura": "I feel like I know her, but sometimes my arms bend back" (because the spirit Laura possessed the human Laura Palmer, but is actually a spirit). I think the "arms bend back" clue was solved with the image of the mother/experiment that showed the arm backwards or on wrong side of body - arm bends backwards is a clue that she is of the spirit world.
There are humans who the spirit Laura looks like: Laura Palmer and Maddie, but both are now dead and I do not believe will ever return. The spirit possessed Laura Palmer - although it completely failed to protect her or destroy Bob 25 years ago.
I believe the spirit Laura started re-entering the picture in the scene outside Cole's room. The spirit Laura will help defeat the Bob spirit and Mr. C - because she is the One. She is trying to get Cole's attention because of his ties to good Cooper and the mysteries of Twin Peaks. Maybe she currently possesses TP or even Gordon himself?
"The Return" can refer to many things, including spirit Laura returning to battle Bob. Sheryl Lee likely has an important part to play later in the series, but I think the human Laura Palmer is gone forever. It will be the spirit created by the Giant who is victorious at the end of the series, not the human Laura Palmer.
I am inclined to agree with Jeffrey here, I think this is exactly what the story appears to be moving towards. I feel that this is a bit of a shame however. I personally don't really like the idea of a 'spirit Laura'. For me, that feels messy and poorly conceived. Just my opinion.
I think this is a pretty good prediction for what is happening. Any ideas about how Laura being dragged upward screaming fits into this trail of thought?
I wondered if it was Laura going through the same process Dale went through (changing lodges, just after the non-existent bit) but seen from another's perspective.