we knew that the ex will be back at some point. And now it's the time after the letter thingy.
Manaka hasn't really turned the page on this past relationship. Even while writting the letter, she remember she has done the same back then.
she never forget truly forget him and the chance are high she'll go to him to check her feeling (and we know how she has been trained by him in the past. Her body hasn't forget).
Ueno, after reading the letter, just remember the memories with the gang. And as weird it can be for some, I see that like fond memories

(Tsubomi

).
I can't see him say yes (independent of the ex thingy that'll happen). He has too much things to resolve for himself and I don't see him be ready to let it go. What he have with the girls is peculiar.
Worth checking on last panel, only Manaka is blushing while Ueno seem chill