Well covering a song is not about making it sound the exact same way it did. I think an artist covering a song can add their own vocal elements to turn that song into a slightly different experience.
I personally think kuu would be able to do a much better version of kanariya then ayu did
Now that was a song in which undeniably ayu vocals grates on everybody's ears, much worse than any kuu ballad can. ayu's very far from a vocal goddess anyway....
On a side note: If such album were ever made, I believe M would already be reserved for the actual Dream cover. Since avex loves to recycle, I don't think they'd waste the chance to use a cover that already exists LOL