I don't know if there's a "right path", depends on what you want or like. I usually go for the church path because of Mira and the possible implications that choosing the demons could have on her (I'm a villain, not a monster!

). As long a one priestess is virgin you can do whatever you want with her others, for the extra option, you should fuck Heather and leave Bridget alone (the point of no return with her is forging the letter, after that doing that don't go to her room). This way, you can talk to Heather about what you found out and when you go to the abbot to accuse someone you'll get a new option: telling him that the murder run away; although I always accuse Antonius because he always tries to sabotage the ritual and make the fight longer than it should be.
If you want the "best" of both path, you can side with Syfa, switch all the butt plugs, and then betray Syfa succefully completing the ritual. This way you can finish in the church path AND corrupt the priestesses. It's "gaming the system" but it's something we can do.
It never occured to me, saving Bianca and Gavina with Tia and Lyvia, I just use silver arrows (I always do that quest after Arianna's quest) or magic if I rushed, for some reason, witchcraft. Next time I'll try going to with Tia and Lyvia and see what they have to say.