Maybe we could focus on the question "why did D so strongly want to tell Elena about the true nature of her relationship with F?" Most obvious answer, of course, is that Elena is her best friend and that she simply wants to tell her best friends about crucial elements of her love life.Now that the cat's out of the bag, so to speak, I think F and D discussing it with Elena may come a lot easier than I had previously thought... more or less..
For a while when .17 dropped, I started thinking about how they would bring that particular conversation up at the right moment. I mean, is there ever a "right moment"? I don't think there is.. especially when it comes to something of this magnitude. .18 gave us something that, honestly, even I didn't see coming after they went through so much trouble to hide it from everybody in the first place.
Even after all of this, I still don't see how they'll be able to just pretend it didn't happen and chalk it up to a misunderstanding. People see what they want to see and that's just how Elena is, IMHO.. She looks at things at face value. So no matter how F tries to sugarcoat it, Elena already has it in her mind that there's more to it than just finding them in bed naked together. In her mind, they had sex and passed out afterwards. It doesn't get much more black & white than that to her. Maybe if they had one of those "Men in Black" memory eraser things, they could zap her with it and go on about their day as if nothing happened.
D slipped up and she'll realize that later. There's no going back from that. She'll feel all sorts of grief and guilt that will make them finally come to the conclusion that they both need to be a lot more careful in the future. At least until they leave Donutistan. Though now that Elena is aware, at least F and D won't have to pretend and hide it from her anymore. So, that in mind, maybe D wanted to get caught? That would certainly be one huge burden she won't have to carry around anymore now that Elena knows.
But maybe a second answer deserves deliberation here. As D is getting more and more corrupted (she knows very well that having mutual masturbation activities with your Dad is not what people would consider innocent) she might get interested in sharing the thrill of the sexual activity with her best friend. Didn't we notice, in the last few scenes showering with Elena, that D was surprised by the feelings that it generated in her?
So I think that the story so far has been preparing us for the possibility that D is gradually opening up for a threesome. That doesn't mean it has to be developed in the very next update. Especially in the D-only route, even the father will need some time to adapt to the idea. (He was so careful to reject Elena up till now, to protect that D is his priority.)
I guess 0.19 will be about this mindchanging process.