Now you are not paying attention to the dogma itself but only to a part of it.
It is by the grace of Christ and the “merits” of His passion that Mary is made sinless. Christ is just as necessary to protect her from sin BEFORE she committed one as He is to protect me from sin after I have committed many.
You gotta read the actual relevant parts of the offishul declaration. Your argument was, in fact, one of the reasons the Dogma was not declared for so long. But then people began to think about how the effects and benefits of the “Work of Christ” are applied in time, and the problem melted away.
God does not protect Mary or you from sin...God protects Christians from the rightful result of sin..