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I don't think the protestant counterargument makes sense.

Mary was born without sin according to the Immaculate Conception. That means she was justified in the eyes of God. However, Adam and Eve both were born without sin and both were justified in the eyes of God, before they lost their justification through the original sin.

Being born without the original sin has no relationship with being able to stay justified during one's whole life until death. As long as someone is alive and is not in heaven, sinning is still possible. The sacraments would still be useful, praying would still be useful, being humble would still be useful even for someone who is justified.

Lazarus, your answer is disjointed, but perhaps I did not distinguish well. Mary was conceived free of original sin and never committed even the slightest sin; in fact, she did not have even any moral imperfections. Several saints were born justified, St. John Baptist, most certainly St. Joseph, the prophet Jeremiah, etc.

Our Lady continued to increase in sanctifying grace by virtue of her baptism and the other sacraments she received, never having lost any of these sacramental graces through sin. Pius XII's Mystici Corporis says that "our Savior as head of the Mystical Body is the 'only mediator of God and men.'" It makes for a very complex distinction to say that Our Lady is co-mediator, which is what I take co-Redemptrix to mean.

It was the same Pius XII in the aforementioned docuмent who specified that Mary was the second Eve, and that she was free from all sin, "original or personal." This settled the question for those Protestants who would erroneously say, "Even if I agree with you that Mary was conceived free of Original Sin, she committed sins later on." Pius XII closed the door on this opinion in Mystici Corporis.

My point was answering against the idea that just because someone is born without the original sin, they would not need the sacraments, or that they were unable to sin at all. 

Our Lady still needed the sacraments because She was still subject to the Old Law (i.e. purification) and also the Church, post Pentecost.  The protestants reduce everything to grace/sin, which is an error.  Our Lady needed the sacraments because of Church AUTHORITY (i.e. baptism is required for salvation and so is the Eucharist, for adults). This is the part that protestants want to ignore - that Christ created the Church with RULES and even Our Lady needed a Savior (so She says in the Magnificat), even if She didn't need a Redeemer (because She didn't have original sin).

Our Mother was capable of sin if I understand correctly the doctrine, she simply didn't sin at all during her life through her own choices, which can be considered to be extraordinary because Adam and Eve were not capable of doing so.

This is not exactly true.  We have to understand the nature of Adam/Eve, pre-sin.  They were not tempted to lust, or envy, or hate, or any sins related to our fallen nature.  They were only tempted by PRIDE, (similar to the angels) because Adam/Eve's nature was in a perfect state.  Lust, envy, hate, etc are sins whereby the emotions/will rebel against the intellect.  But this was not possible for Adam/Eve because their intellect/reason was IN CONTROL of all their human faculties.  The only way for them to sin was through their intellect/reason, which we call pride.

In the same way, Our Lady could not sin by way of her lower nature, because Her lower nature was not corrupted.  Therefore satan wouldn't tempt Her in this area; it was off-limits to hell to tempt Our Lady, which is why She is so unique and holy.  And because the only way She could've sinned was through pride (i.e. intellect/reason), and She didn't, this is why the devils hate Her so much, because She is literally, the most-humble person who has ever lived (and the most humble of all creatures COMBINED) because not once did She ever sin through pride, but her humility was constant, perfect and pleasing to God in every way.

So, yes, in theory Our Lady could've sinned ONLY through pride (but She did not).  But it's incorrect to say that She was tempted to impurity or anger, etc.  Neither was Our Lord tempted to any of these sins, because His passions were ordered and controlled (just like Our Lady's) by reason, as God originally designed Adam/Eve.