Jackie is mentioned briefly in Act 2 a couple of times. We do not see her as an adult though, only as a memory/ghost, and she is still played by a child (Sarah Hyland).
The set for Act 1 was mostly the living room in the Beales' East Hampton mansion. They did transition to outside for 2 songs (Marry Well and Goin Places) and it also changed a bit for Will You?.
In Act 2 the set changed several times - it was Big Edie's bedroom, Little Edie's room, outside, the front of the house, etc. But while the first act was very bright and grand, the second act was obviously very dark. But in Act 2 we never saw the same room we saw in Act 1 (I am pretty sure - it has been almost 3 years since the closing performance).
As for the Sarah vs. Erin debate (for the millionth time), I only saw Erin but between Sarah on the recording and Erin, I much prefer Erin. She has a stunning voice and gave an absolutely gut-wrenching, heartbreaking performance. I'm sure Sarah did as well, but I did not see her.
"This table, he is over one hundred years old. If I could, I would take an old gramophone needle and run it along the surface of the wood. To hear the music of the voices. All that was said." - Doug Wright, I Am My Own Wife