There is no way to have a happy ending for Mayrina's story, and she implies she is going to Baldur's Gate so you may encounter her there again.
Oh I don't know... Still alive, still safely carrying her child, and on her way to Bg (with her safely ambulatory zombified husband), where she might find someone who can raise said zombified husband properly provided she can scrape together the means to pay for it or work out an agreement... and if you clear out the swamp first, but don't talk to the witch or enter her house, and only then go back and do the road-side encounter, and side with the idiot brothers, they survive as well... I'd count that as a 'positive as you can hope for' outcome for now...
Not to be that person, but it is obvious with the Larian clues in the Hag lair that Mayrina will run into the Hag again in the city and still have her child taken from her.
