If God could simply declare mary to be free from sin.. then why bother to send Christ at all?? He could simply remove the sin of all men ..Bingo no need for the cross
May I be permitted one small "Arrggghhhhhh!" of frustration?
The teaching is NOT that God simply declared Mary to be free from sin. It is because of the "merits" of Christ:
Declaramus, pronuntiamus et definimus doctrinam quae tenet beatissimam Virginem Mariam in primo instanti suae conceptionis fuisse singulari Omnipotentis Dei gratia et privilegio, intuitu meritorum Christi Jesu Salvatoris humani generis,...I THINK that once again, the hurdle is not only about the Gospel but about "time and eternity." We think that it is "self-evident" (clearly not in the sense of "obvious") that eternity is something which may contain temporality but which is outside of it, in such a way that all times may be said to be "now" in eternity.
"We declare, pronounce, and define that the doctrine which holds that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, ...
SO we are not bound to think that the effects of Calvary and of the work of Christ generally are bound to happen only "afterwards."
ANd because of the hypostatic union, we think that the sacrifice of Calvary is both in time and in eternity. So that its effects especially may ripple in every direction in time, not just forward.
Sure from OUR temporal point of view, the conception of Mary was 'before' all the "work" of the Incarnate Christ. But to the One to whom all times are "Now", what does "before" mean?
As always, this is not meant to persuade. I would, I think, pose the problem the same way even if I weren't Catholic. I'm just trying to present the shape of the argument and why, in light of that shape, your objection doesn't address the question.