http://www.udayton.edu/mary/questions/faq/faq17.html
Why do Catholics pray to Mary?
For Catholics, there are different types of Marian prayer that reflect different intentions. Reciting the "Magnificat," for example, is a way to praise God as Mary did. By reciting the "Angelus," you commemorate an event in salvation history, one in which Mary had a major role. Such commemorations are also made by the entire Church in the liturgical cycle. At Christmas, for example, the faithful recall Mary's role in the birth of Christ.
Prayers invoking Mary's intercession for your own personal intentions are in a separate class, arousing controversy since the time of the Reformation in the 16th and 17th centuries.
The practice of invoking saintly people for their intercession before God in union with Jesus has roots in sacred Scripture. In this regard, Matthew 18:19-20 refers to saints on earth, and Revelation 18:20 refers to saints in heaven. In the early centuries of the Church, veneration and invocation was offered to those martyred for Christ. The practice derives from the doctrine that the saints are united with Jesus in one mystical body (Rm 12:5).
The practice of calling on Mary for aid also appears to be very old in the Catholic Church. An ancient testimony of confidence in Mary is the prayer Sub Tuum, which historians place in the third century:
We fly to your patronage,
O holy Mother of God;
despise not our petitions in our necessities,
but deliver us always from all dangers,
O glorious and blessed Virgin.
(Dictionary of Mary, p. 143)
This practice became an area of significant disagreement between Catholics and Protestants. After the crisis of the Reformation, the Catholic Council of Trent repeated traditional Christian teaching on the intercession of the saints, which applies preeminently to Mary:
The saints who reign together with Christ, offer up their prayers to God for men; ... it is good and useful to invoke them suppliantly and, in order to obtain favors from God through his Son Jesus Christ our Lord who alone is our Redeemer and Saviour, to have recourse to their prayers, assistance and support.
(Theotokos, p. 188)
The Synod of Jerusalem held by the Orthodox Church proclaimed a similar position about a century after Trent. Protestants counter that reliance on Mary and the saints detracts from reliance on Jesus Christ, who is our 'one mediator':
For there is one God, and one mediator also between God and men, the man, Christ Jesus, who gave Himself as a ransom for all, the testimony borne at the proper time.
(1 Tm 2:2-5 - New American Bible)
A thorough examination of this topic by the Lutheran-Catholic Dialogue Commission may be found in The One Mediator, The Saints and Mary (Augsburg Press, 1992). An excellent contemporary Catholic reflection on the subject is found in chapter 3 of Pope John Paul II's 1987 encyclical, Mother of the Redeemer. A small subsection of paragraph 38 of that chapter is as follows:
The Church knows and teaches with Saint Paul that there is only one mediator: "For there is one God, and there is one mediator between God and men, the man Christ Jesus, who gave himself as a ransom for all." (1 Tm 2:5-6).
The maternal role of Mary toward people in no way obscures or diminishes the unique mediation of Christ, but rather shows its power [Vatican II Constitution on the Church, # 60]: it is mediation in Christ.
Catholics do not pray to Mary as if she were God. Prayer to Mary is memory of the great mysteries of our faith (Incarnation, Redemption through Christ in the rosary), praise to God for the wonderful things he has done in and through one of his creatures (Hail Mary) and intercession (second half of the Hail Mary). The latter is addressed to Mary not as to a vending machine but a support person helping us to discern the will of God in our lives. Mary is a volunteer, highly recommendable and recommended, but not a mandatory and inescapable passage.
And to follow up on the last part of your question (I think it got lost in all the kafuffle.

Matthew 27: 11-26 (from NIV)
11Meanwhile Jesus stood before the governor, and the governor asked him, "Are you the king of the Jews?"
"Yes, it is as you say," Jesus replied.
12When he was accused by the chief priests and the elders, he gave no answer. 13Then Pilate asked him, "Don't you hear the testimony they are bringing against you?" 14But Jesus made no reply, not even to a single charge--to the great amazement of the governor.
15Now it was the governor's custom at the Feast to release a prisoner chosen by the crowd. 16At that time they had a notorious prisoner, called Barabbas. 17So when the crowd had gathered, Pilate asked them, "Which one do you want me to release to you: Barabbas, or Jesus who is called Christ?" 18For he knew it was out of envy that they had handed Jesus over to him.
19While Pilate was sitting on the judge's seat, his wife sent him this message: "Don't have anything to do with that innocent man, for I have suffered a great deal today in a dream because of him."
20But the chief priests and the elders persuaded the crowd to ask for Barabbas and to have Jesus executed.
21"Which of the two do you want me to release to you?" asked the governor.
"Barabbas," they answered.
22"What shall I do, then, with Jesus who is called Christ?" Pilate asked.
They all answered, "Crucify him!"
23"Why? What crime has he committed?" asked Pilate.
But they shouted all the louder, "Crucify him!"
24When Pilate saw that he was getting nowhere, but that instead an uproar was starting, he took water and washed his hands in front of the crowd. "I am innocent of this man's blood," he said. "It is your responsibility!" 25All the people answered, "Let his blood be on us and on our children!" 26Then he released Barabbas to them. But he had Jesus flogged, and handed him over to be crucified.
But the fact of the matter is that the Blood is on OUR hands and washed OUR sins away. He didn't die for Jewish people or for Roman people or for Pilate alone. He died for us.