This is kinda hard to explain, but there are 4 different routes you can pursue after corrupting Elena inside the cabin, them being:
1 - Sera's route - Fight with her at night and then break her curse.
2 - Cecille's route - Train her powers.
3 - Elena's route - Meet and pass the night with her.
4 - Erin's route - Meet Lavinia in the evening and train Erin's feminization.
Currently, you can finish those routes and continue playing on the same save because the game is incomplete beyond those routes ending, but on the next update onwards, it will only be possible to complete ONE of these 4 routes (No, you're not locking the romance out with anyone, if you're pursuing the harem ending) as they all lead to the climax of the arc (it being Erin and Mars battle). Alba's death only happens when you progress Cecille's route (her powers training) to the end. If you progress any other route, Alba's death is never going to happen because you chose another route over Cecille's one.