Thanks for starting out easy. As Catholics we believe that death does not exclude one from membership in the Church. The Church is composed of all believers, both those here on earth and those who have died in Christ. Just as we can ask those our brothers and sisters in Christ here on earth to pray for us, we also can ask Mary, and the other saints, who are now in heaven with God to pray for us. Writings and archaeological evidence from the earliest days of the Church (when Christians still met in secret before 330 AD) show us that this practice is an ancient and accepted practice back to the time of the Apostles.
One of the problems that non-catholics have when they see Catholic practices they are not familiar with, or hear Catholics talking about something like praying to Mary, is that the Catholic Church is 2000 years old, and some of the terms we use are derived from older language sources. If you read the King James Version of the Bible, or Shakespeare, you will here old English, terms, if you read something like, “I pray thee good sir” It is not a form of worshiping that person, “pray the” is an old way of saying “ask you”. So when we pray to Mary, we are asking her to pray for our intentions along with us.
Another problem that Protestants have with the Catholic practice of the intercession of the Saints, is that they have been told that Catholics “Worship” Mary, the Saints, and Statues. Catholics believe that only God is worthy of Worship, (The technical theological term is Latria) the Saints because they have lived lives of great devotion to God, and are great examples of Faith, and are members of the Church now in heaven, are honored, but not deserving of Worship, (the theological term is Dulia) And Mary, because she lived a life of such great devotion to God, and gives us an example of one who gave total devotion to God, is honored as the human being who gave the greatest of devotion to God, (the theological term for the great honor she is given is called hyper-dulia)
Any graces we receive through the intercession of any other members of the Church, be they someone here or in heaven are given by God. The Church does not command that we pray to Mary or the Saints, but recommends it as an acceptable way of honoring God by honoring his saints. I’ll answer the other questions separately.