No, most likely you have to be Orlaith's sub, then you can put the idea into Grainne's head. I'm 85% sure it's not going to be just you walking in on Orlaith and Grainne getting it on if you never took her up on her offer. Idk I don't think there was a doc moreso Wsan spitballing ideas,
As for domming Orlaith I don't think it's a corrupt character only thing. I think it's like raping Infrith, just something for corrupt characters to do. If I remember Grainne and Doireann aren't going too happy if you go through with it which is why I dubbed it corrupt
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Wsan ain't the most talkative guy, so you have to take 4chan for their word, I'll keep browsing to see if there is more irrefutable proof