I have some random theory to share
first of all about the promise girl, I think Onodera and Chitoge still have 50 : 50 chance, but why?
Chitoge's point : well, you know she was the main female protagonist and the author on her side obviously
Onodera's point : the only girl who have a chance to stand against Chitoge
1. Her key is the one which very similar with the promise girl (even though it just a silhouette and changes every chapter I read)
2. Yui did not mention her name early in the class when first time she came (she mentioned kirisaki, marika, tsugumi and even shu), she just gave her a glance and smile (I sense a smell of inferiority)
3. In Yui's perspective Raku is dating Chitoge and have feeling towards her, her statement about "When you know about the promise girl, do you still able to love her?" maybe means that promise girl was not Chitoge.
Why not Yui? Because Raku fully aware that she was his precious big sis when he was a kid, I don't think he would makes 'lets married' promise with a big sis
Why not Marika? despite who is the promise girl, I still hope Raku will choose her but I don't think she was the promise girl because it looks like she 'remembered' but pretend to 'forgot'. I don't see any reason she pretend to forgot if she was the promise girl.
despite of that bunch of random thought, there is still a big chance that Yui's key is not a real one and the author still have 1 last key holder to show.
this manga (maybe) has 3 years arc afterall and we even have not finish the 2nd year, so there's still a big chance this last theory to be true.