"My spirit hath rejoiced in God my Savior."
To rejoice in her Savior, means she acknowledges she is a sinner in need of salvation, i.e. needs a Savior to save her from her deserved destination -- Hell. (Where we all would be BUT for the saving Blood of Christ.)
To be saved implies there is something she needs to be saved from. If she was sinless, she would not need the Savior and therefore her spirit would have no need for rejoicing.
You are really stretching it here. You make me laugh.
Think carefully about your logic. How do Catholics think Mary became sinless? By her own effort? Because she was so good-looking? No, by the foreseen application of the merits of her crucified Son. That's dogma, a Catholic who denies it is properly called a heretic.
So, if she calls God her savior, it follows that she needed saving and was in fact saved. It does not follow that she had necessarily committed personal sins. Babies haven't committed personal sins, yet none of them can get to heaven on their own merits (to say otherwise is the heresy of Pelagianism). Catholics are required by their faith to believe precisely that her sinlessness was a gift from God, that is, that it was part and parcel of the grace that saved her.
And none of that has anything to do with praying for the Poor Souls in purgatory, which was the original topic of this thread.