there no official walkthrough, only player compiled data. All avaiable at the bottom of the OP.
If you see Rifina getting raped, then you took to much time (you won't be able to recruit her later). In this game, in part 1 (until Rifina rescue) progressing in character recruitement related quest make time move forward. Each forward even mean Rifina get more molested. If she's raped, it's "too late".
However, the game don't gave you enough time to do everything in one run. Therefore you have to chose what to do in a playthrough. Knowing that any character recruited in a previous playtrhough will be avaiable from the start in alter playthrough... but you won't have acces to character related quest / event if you didn't recruit them in you actual playthrough.
But yeah, you need to get to that dungeon and rescue her to get to part 2 of the game and officially have Deanarus and Letina as avaiable party member.