Rabiane even making it all the way through had to be devine intervention/luck. Maybe thats what the OVA was trying to establish?
Also love your art btw, so hot. Will there be more?
More or less, divine intervention. Remember that Otherworlders have limited influence in the Human World and are not schemers creating an entire mess to attract their ideal vessels. Any of the game overs before the Priest's Nightmare can happen to Rabiane, and when they do they don't work in favor of the ultimate goal of the Observer -to be born through the perfect vessel. Rabiane becoming livestock, a parasited nun, a brainwashed sex slave for the cultists, or a seedbed for the slugs will not lead to the birth of the Observer; so he sends visions to tempt her, torment her, but also to allow her to make her way to its personal ground on top of Kessaria's Cathedral.
The Father simply waited for someone to arrive. If they can't perceive it, they get mutated without a second thought, same if they lose to its initial attacks when revealed. It only attempted conception when the sisters proved dangerous and powerful enough. Meanwhile, the Observer provided Rabiane with visions of death and pleasure from the beginning, accepting what we saw in the OVAs as canon; they helped her arrive where the Observer needed her, with the express purpose of forcing her to have sex with the special slug so she could fight it. That's why I rated the Observer higher in my joke dialogue.