Your right yuri sex is great. I mean nothing was more satisfying than Touko and Yuu's sex scene after the 45 chapter journey that was Bloom into You. However I'm just asking for any solid proof that there is more to Matsuri and Miss Sunflower's relationship than the close bonds of a student/mentor.
You're quite right--that's a question this series has posed from the very beginning and has yet to resolve.
On Matsuri's part, I don't think there's ever been any doubt that she has always had a full-scale romantic girl-crush on Miss Sunflower--she thinks she's cute, she swoons at Miss Sunflower treating her as someone special, she compliments Miss Sunflower's legs, etc. She likes her that way.
On the other side of the equation, in the beginning of the series I thought that Miss Sunflower was just treating Matsuri as a little kid and was entirely dismissive of Matsuri's romantic feelings. But as the series went on and we learned more about her, we saw that she keeps everybody at arm's length, emotionally speaking, and that she in fact has let Matsuri get closer to her than anybody else in her life. Furthermore, her relationship with the first Miss Sunflower strongly suggests that she (not to put too fine a point on it) likes girls.
We also found out that the age gap between the two isn't quite as large as it seems. (I forget who did the math, but I think it came out to something like 23-17, maybe?) The apparent age-gap was exaggerated by their respective body sizes and by the fact that Himawari-san acts a lot older than her age while Matsuri comes off as considerably younger than she is.
I've been arguing that Himawari-san's closed-off psychological state, which at first seemed like calmness and maturity, is something that the series has been positioning as a thing that, gently and gradually, needs to change and is changing, primarily because of Matsuri. What the end state of that process will be I have no idea. But as with Living With My Brother's Wife, the quality of the series itself has convinced me that I'll probably be OK with however it comes out.
No fair using a calm logical break down, of the plot and the characters' personalties. lol