Hi clarem,
So sorry that you haven't had any posters.
I'm so glad to hear that it's taking less time for her at BT.
Did she always drop naps earlier than usual? Asking because she may be one of those that drop nap earlier (around 2-2.5 as opposed to 3 or 4), and sleep better at night. If she starts to take long to fall asleep again, you have the choice to shorten her nap even more, or push out BT later.
For sitting up in bed, I would PRACTICE as much as possible during the day. I would model it, make a game of it, and have her try it everywhere... on the floor, on the couch, even in her bed during the day. Be very specific, yet simple with instructions. "Arms out and down, head down, roll over, tada!" Let us know how that goes on. After a couple of days, when she sits up at night, go to her and ask her to put "arms out, head down, etc..." until she can do it independently. Eventually, wait longer to go to her and she how she does. Hope that helps.