If there is "nothing binding" If there is an "error" I would like to know which is error, That only Christ could keep the whole law perfectly, or that Mary also kept the law perfectly?
Only one can be true ...
Repeating that out of context fallacy lends no support to you argument.
"Only Christ is sinless on his own account. Mary was rendered sinless on account of a prevenient action of God, applying Christ's future merits and perfection to her. Mary did no work to make her a sinless vehicle for God-in-the-flesh. It was purely on account of God's grace, and the work of Christ, that Mary was made sinless." (1) if your meaning is "sinless on their own account, the answer is: only Christ.
(2) if your meaning us "sinless by the work of Christ, not by innate righteousness but in order to be equipped for the unique role of being mother of our Savior," the answer is: only Mary.
Note that Mary, while free of in, is still "short of the glory of God." So her righteousness is entirely derivative, and is not symmetrical or commensurate with His. It is, as has been poetically expressed, like the reflected brightness of the moon in relation to the self-generated brightness of the sun.