You're advancing Dunn's argument which is a weak argument to justify the Immaculate Conception.
God is capable of doing whatever He wants. However, He will not contradict Himself.
Where did you get this Divine information?
IF one reads the Bible one will realize that everyone born of man and woman since the Fall is a sinner.
There are no carve outs for Mary in this regard.
You mean God wasn't capable of having Mary free of original sin?
The better question would be is God capable of preserving Christ in the womb from the effects of sin with the need of Mary being sinless??
Indeed, during Christ's entire physical life on earth, He was surrounded by sin 100% of the time in every human he met. He remained Holy. He was more than capable.