To unlock the second scene, you must first complete the first scene; to unlock Stela's first scene, you must not bring Rosette to the experiment
Viewing post in Thou, Eternal Everbloom (18+) comments
I'm not the game's creator, but if you've taken a moment to read, before entering Stela's house, the game asks, “Do you want to bring Rosette with you?” and I think that implies that either something good or something bad could happen, depending on what you prefer.
As for saves, you can now import other saves if someone shares them on Android; you can do the same on PC, but if not, you can start a new game and skip everything up to version 0.1.4.3, which is already a big improvement.