The whole situation is kinda...all over the place
I mean, it's true that Riri should have gotten the message after many rejections, but "love is blind" and, IMO, from her perspective, the teacher was kinda "dismissive"(?) of Nanjou's feelings (so, to her, she still got a chance). Yeah, yeah, the teacher might have been doing that out of tsunderism or from the fact that Nanjou was still a student long from graduating. So, now that it's not so far away, we saw a glimpse of sensei's true feelings. So, in the end, Nanjou was right to keep pursuing the teacher.
Now, Nanjou probably already know that the teacher actually shared her interest, so as I said, she was right to keep shooting her shots. So, what about Riri? All the times Riri confessed her love, Nanjou told her she was in love with the nurse, so it's kinda on Riri to not get the message....but, if it was ok for Nanjou to keep pursuing Sensei, why should it be a fault that Riri kept pursuing Nanjou? Sure, Nanjou could have been more firm in just one of her rejection, to clearly draw the line...but she didn't want to hurt her junior (as she said)...everyone would be hesitant of hurting someone else's feelings, so can we blame her? I think not.
To love is to hurt and get hurted...if 2 people are in love, there surely is at least another person who's hurt for not being one of the two.
To put it simply, we got this weird love situation where: (-> = love)
Sensei-><-Nanjou<-Riri(<-Uda)
So, in my opinion, when the topic is about feeling, there's no right or wrong and nobody is at fault for loving somebody
The only thing that I'd like, is for author to not use Uda as a cushion for Riri, to write all the character involved mature enough to accept how the thing developed and expecially make Riri AND Uda mature enough to understand that Riri might need a bit of time to clear her mind before trying to pursuit a new love.
And also to return to the diabetes-inducing main couple lol