Not so, because if it were in a Knight there would be no imbalance.
Unless the Knight was transformed.
However you are also wrong, the Mother's would know if it had been in a Knight, because there would be no imbalance.
Unless the Knight was transformed.The Queens would know, because there would be no imbalance.
Unless the Knight was transformed.Unless it was transformed which it was.
See? You know this. So what even was the point of those three sentences?
Now you'd think it would be hard to transform a Knight once the mantle is established into a statue.
Why? I don't think it would be hard for a Summer Queen to do to a Summer Knight anything she wanted. It's not like Lily was known for her strong will, and she wouldn't have the first clue how to use the power in the mantle if she wanted to--which, given that Aurora was her friend and almost certainly didn't tell her what she was planning, she almost certainly wouldn't have.
And I think turning a Knight into a statue would be much easier than trying to stick the mantle into a statue. I seriously doubt the mantle could be put into a statue that wasn't also a person, since if it could be then there would be no reason for Aurora not to just do that--and given that the Lily-statue was so much a statue that even a normal counter-spell from Aurora (who cast the spell) wouldn't un-statue her, and an Unraveling from Mother Winter was needed, it seems far more likely than not that the mantle would have treated someone trying to put it in statue-Lily as someone trying to put it in a statue, not a person.