the big bad is the goddess, she's orchestrating everything behind the scenes, and lufasu was having difficulty working without the goddess interfering, so she created an avatar in another world, made dina wipe her memory, and got locked in hyperspace. hence, when lufasu returns, the whole "mind switch thing" is just with herself, but the goddess has no clue, meaning that lufasu can work without interference. also she's a total nerd if you didn't pick that up, so the whole forced speaking thing is there to keep her from completely embarrassing herself I believe lol