Looking forward to seeing these two on NHK Songs
I'm a bit late to the discussion, but in regards to a possible Ringo version of 暗夜の心中立て: I don't think Ringo will necessarily sound worse than Sayuri, but he version will probably be different. I expect that if Ringo does a self-cover of this song, she will change it up a bit and sing it in a way that suits her style of singing, rather than trying to imitate Sayuri.
It's also possible to compare Ringo and Sayuri already, since both of them have sung
Kurumaya-san in the past!