When I said something pushing, that was in the context of an external force like the gods bending her will to fit. Throughout the story so far, there are signs that something outside the girls themselves and Sensei is acting to push the girls toward him, making them start to feel something for him, whether that's actual affection or lust, to drive them toward allowing him to get in their pants. I'm not getting that with Tsubasa...yet. I am seeing the angle you're talking about, though, and that's where I'm seeing the beginnings of something that could evolve into what pushes her into sex with Sensei. I just think its too early to say she's ready yet. I would say within two to three periods of development, or releases with either her directly or with her daughters to put it another way, we should start seeing the signs of approaching sexual content and then maybe one more after that to actually get it. Keep in mind this is all theory, a prediction as accurate as a hurricane track for a hurricane that just formed off of Africa at best.
The way I see her forwardness, that might be a front to hide her true feelings. I get that the rich life might not satisfy her and may make her bored, but look at Touka, this family clearly has little interaction with men outside the one man they have in common, the father of Touka and Tsukasa. She may be putting up a front to hide nerves and uncertainty around the possibility. We've seen such tactics before, hiding their true feelings, I mean. That worry about her husband, that's another reason I don't believe she's ready. I think we'll end up seeing something change there and that would be a sign she's getting ready for sexual content.
You've got another point with Sensei himself, he's been changing. We'd already have a threesome if this was still day one Sensei, that one wouldn't hesitate at the offer, but I can't seem to get a read on his willingness as he is now. He's just so...uncertain of his own actions. That provides an obstacle for all of the options in that specific set, not just Tsubasa or the other seemingly wrong choices, but even the Ayane choice. We'll have to see how this plays out.
I honestly don't see Yumi being convinced, especially under the current circumstances. Not only is she still a bit on the hostile side over what Sensei did and allegedly did, but she also knows he's going out with her best friend and she does not want to hurt Chika like that. A betrayal that big would be an extreme blow to Chika and she knows it. That's where the 'possibly setup for the future' theory comes in with the flags. We don't know what the circumstances will be in the future of the story, maybe something will clear the way for Yumi by the time she is actually willing to commit to giving herself to Sensei. I just hope whatever it is isn't Chika's death.
Technically, yes, and one of the rare cases where it did happen. I'm not saying the first scene for a character can't be a threesome, I'm saying it's so rare that it doesn't fit the pattern. It could happen, but the first scene has always been more likely to be a one on one. As far as those Nodoka appearances, I am specifically talking their own scenes or scenes where they actually participate. Maki would be one of the few times where a threesome where she was part of it was technically her first scene.