a) Can Mary hear everybody everywhere as God the Father and Jesus can?
b) Does Saints really hear our prayers?
The Blessed Mother is a finite creature, whereas God the Father and Jesus are omnipotent God.
Moreover, because Blessed Mother continues to live in Jesus and the Father in heaven (see John 15:1-11), and because she is not constrained by the limits of time and space as we are on Earth, Mary is blessed to be able to receive and intercede for the prayers of the multitudes who beseech her intercession.
Can They Hear Us?
One accusation made against it is that the saints in heaven cannot even hear our prayers, making it useless to pray for their intercession. However, this is not true. As Scripture shows, those in heaven are aware of the prayers of those on earth. This can be seen, for instance, in Revelation 5:8, where John depicts the saints in heaven offering our prayers to God under the form of “golden bowls full of incense, which are the prayers of the saints.” But if the saints in heaven are offering our prayers to God, then they must be aware of our prayers. They are aware of our petitions and present them to God by interceding for us.
Some might try to argue that in this passage the prayers being offered were not addressed to the saints in heaven, but directly to God. Yet this argument would only encourage the fact that those in heaven can hear our prayers, for then the saints would be aware of our prayers even when they are not directed to them!
In any event, it is clear from Revelation 5:8 that the saints in heaven do actively pray for us. We are clearly told by John that the incense they offer to God are the prayers of the saints. Prayers are not physical things and cannot be physically offered to God. Thus the saints in heaven are offering our prayers to God mentally. In other words, they are interceding.