Clothing in TV always tells a story. It ties people to their surroundings, it ties them to other characters, or conversely it sets them apart from others in an effort to make them “stick out.”
ONCE doesn’t use this as much as–say–“Mad Men” does (about which essay upon essay could be written) but it does use clothing to tell thematic stories.
I can’t speak for all your bullet points, but I can address a few
3. She wears her red leather when she’s working primarily as the “savior.”
Both yes and no. The red jacket is, to quote JMo herself, “Emma’s armor.” It’s her protective shield against the world because Emma is a total “walls up” kind of girl. This a layer of clothing that keeps her isolated and protected from the big bad world that has hurt her so often. So for example, in 310, we see Emma contemplating what is wrong with Henry while she sits at the beach where his castle used to be. She’s worried about her son, worried about why he hasn’t called (hence playing with her phone), and worried about the Savior not being able to “take a day off.” It’s the first time in the whole of S3 that we’ve seen the red jacket come back. Because right then, Emma needs the comfort of her walls.
4. In the new photos Emma is wearing plaid pants and a red wool coat–a kind of softer “mom” version of herself.
When those pics of Emma wearing those *hideous* pants came out everyone said, “what on earth!! Emma Swan would never wear those!” And now we know why. She’s not the Emma Swan we know. She’s a different Emma Swan. She’s going to wear things and do things and say things that OUR Emma Swan wouldn’t do. That’s the price of her cursed self.
5. She’s back in the red leather coat when she, Hook and, Henry leave for Storybrooke. Presumably to “save” everyone.
Most likely yes. She’s OUR Emma again by that point. Which means all the pain of being away from her family has just come back, her walls are going up, she has to be tough as nails Emma Swan with her protective armor again. So out comes the red leather jacket.