The threesome is a one time thing, and Sensei explicitly can't have feelings for the other girl afterwards. Even for her best friend, who she knows has a crush on him. She wouldn't be okay with him dating other girls even if he was honest about it.
The phone is helpful yes, but Chika passed up a mansion. If Sensei stopped offering emotional stability by revealing himself to be a lying scumbag, she'd accept some extra financial hardship. She was going to pay for the plan herself in the first place before Sensei swooped in.
Hope actually does want the girls to be Akira's toys. He just needs Akira to be less nice about it, and will take hostages if it makes Akira play his role correctly. I have no idea what the other gods want, since they don't have a priest lecturing Sensei about their objectives.
Also, I replayed the event where Chika talks about the threesome, and apparently getting Tsubasa and Chinami as options needs certain flags? Tsubasa makes sense since you can miss her catching them have sex in the mansion. But what unlocks the Chinami option? I figured it was just a joke choice.
For now maybe, but that's still far more than virtually any other girl would be willing to handle, (besides Kirin, maybe Noriko, and Miku). Chika has also been somewhat supportive of Sensei and Yumi in the past, and seemed to imply she'd be willing to share him with her before, since they mean so much to her. I think she wouldn't be too adverse to Sensei dating others if she had a say in it.
A phone plan from your boyfriend and living in a mansion with your adopted mom are two very different things. Sensei was trying to make her life easier, while Tsubasa is trying to change her life. I think she's far too prideful to accept help out of pity. It's also not just the financial part that matters. He's been there for her family, both Yumi and Chinami, and this phone plan was for them as well. Add on the sex, the memories, the happiness, and whatever else, I'll be surprised if Chika ever actually breaks up with him. I think hurting Chinami or worse would be the only thing that could turn her completely against him.
HOPE wants Sensei to use them, but I don't think it ever expressed it wanted them to actually be Sensei's in anyway beyond being his seedbeds. Sensei might as well be HOPE'S toy as well at that point. This is not a scenario I'd be into tbh.
As others have said, I'm guessing it's the bathroom scene. Which... kind of worries me for the future. In this game, I'll be a bad uncle, maybe a bad homie if there was any benefits, but a bad papa is crossing the line... At least for me.
