giả thuyết này cũng hay
The biggest problem isn't promises but the matter of feeling. As long as Raku is still in love with Onodera, the truth about marriage promise is meaningless.
This chapter reveals that Tsugumi was also a part of the kids. This is a very important information but I'm sure Tsugumi isn't the promises girl. She doesn't have a single memories of the promise, she doesn't have key, she doesn't know the story which Raku read to the promises girl. HOWEVER!
There is a very HIGH chance that Tsugumi was the "boy" who made promises with Onodera when they were child. That explains why Onodera also have a promise with a boy but doesn't remember anything after that.
Also it's impossible for Marika being the promised girl because she doesn't know the story Raku once read. If she was, there's no reason for her to hide the truth from Raku.
And that leaves Chitoge. She remember the phrase, has the key, and more importantly, has the story written in her diary. The reason Marika want Raku to break up with Chitoge probably because Chitoge is the promised girl.