As it turns out, the attempted murder is faked by Toyoko Busujima herself, planned together with Eiko Fuda, Kyoko Ugajin, Yuko Suo.
It is virtually impossible for someone to plant a cabbage core inside the monja to choke Tyoko and murder her, as Monja's ingredients were all chopped up during the time that it was made, meaning that it must be planted the moment Toyoko was able to take a bite and thus only she herself is capable of the crime.
Her three friends all share the same filmsy motive for the "murder": Tayoko stealing each of their boyfriends durings the highschool, all of which happened roughly 60 years ago.
Motive
The four ladies want to involve Kogoro and the police in this particular event to promote the "Tokyo Barls Collection" and cheer up all the elderly people in the area. Despite recieving some serious scolding from Inspector Megure, the ladies succeed in involving the media in the situation through their public apology. Their fame also skyrocket after being interviewed by Yoko Okino, making the Collection a great success.