answers_to_first_three_questions

1) No, Garrigou-Lagrange distinguishes between "acquired virtues" (those learned by going through life), and "infused virtues", which come from being in a state of grace. He spends most of the chapter talking about infused virtues, and how they differ from acquired ones (even ones that share the same name, like prudence). This is one example among many of how Christianity has both a natural and a supernatural component.

2) No, sanctifying grace is something God gives us to enable our salvation. A supernatural gift that we did nothing to deserve. It is a prerequisite to make it to heaven. One might say we come by grace with the help of the Holy Spirit, but the Holy Spirit is not grace itself; it is God.

3) Adam and Eve were created in a state of grace, but due to their actions lost that state, and the gift was not restored until Christ's sacrifice for us. As such, it is likely that few people in the time of the Old Testament ever attained infused virtue: it would have only been those blessed by God (not unheard of). But per answer 1, people still had acquired virtue, which is purely a product of the natural world (expounded upon at great length by Aristotle).