When Emma brought Marian back it obviously didn't change the course of the past of any of our characters theres just consequences of her being around now for them.. no different than if Snow's curse had scooped up Marian and dropped her in the present. For all we know Marian escaped the Queen's jail and was living in the Enchanted Forest and just couldn't find Robin the first time around. Maybe they were destined to run into each other again someday anyways.. maybe they're the type that were only supposed to be separated for a while. Overall though- The show is VERY loose about the changing time-ness... Can we count all the Black Knights who were experiencing life differently from one time around to the next? In many many circumstances, Black Knights are treated more like props than actual people.

Anyone with a conscience would've done what Emma did, and Regina knows that. She knows she's being selfish but lets face it, she must be sick of losing her men to this that and whatever. Emma however i agree is being dumb, but not intentionally. Hook is giving her attention- positive attention- and she doesn't know how to deal with it so she's taking whatever distraction comes along. She's blowing the "guilty about Regina's situation" out of proportion and holding on to solving that problem because she doesn't know how to be content. People with low self esteem don't know how to accept a compliment- likewise someone who's lived rough and alone and independent doesn't know how to settle down and let someone else in. Its easier to keep "working".