I'd disagree about Riri. Besides Karin, only Riri has been featured in almost every chapter (I think she's appeared in more chapters than the other ladies). Honestly, if it wasn't for this last chapter Kaoru would be close to last place.
I do like Kaoru but to me, she doesn't feel like she has much of a chance. I think the issue is her personality doesn't fit Mei's that well. It could be her overly aggressive attitude or the sense she represents but I don't see them working, especially when compared to Karin.
I do agree that the last page did give ntr vibes. I'm also curious how Mei will react now that she's been kissed twice. I can see her comparing and contrasting both of their kisses and thinking about how she felt for each. When she kisses Karin during the play it'll be interesting, since it'll be the same setup as her kiss with Kaoru here.
I agree. I think the "positions" of the ladies are determined by how Mei feels, reacts, and treats them. It doesn't feel like Mei really lingers on any one of them specially compared to the others yet. Yeah, she has some physical contact but she reacts in that moment and then easily shifts into fawning over the next girl with no issue. I don't personally think anyone is on a distinct level yet. Some have more contact but that hasn't led to Mei's feeling more secure or attached or confident in her feelings.
I also agree with the Riri push. If any of the author's decisions seem suspect regarding who might be a "favorite" it would be Riri's constant appearances and being the first (even though she hasn't been able to do much). But it's also the fact that she's been positioned in the most sympathetic way and continues to be. The fact that she's given less to do so far might indicate that when it's her time it's the latter half of the series, and that might say something about her role. Something like "all the others get their shot and yet Riri is always there." But we'll see. There's a harem route obviously possible too so there are a lot of possibilities and nothing has been fully tipped off yet.