To sum everything up using my finite knowledge:
-Roman Catholics believe that Mary plays a huge role in Christianity (she does. She accepted the request of Gabriel the Archangel, who was sent by God, to conceive Jesus.) They do not worship her. Rather, they praise her for her accomplishments. This also applys to Saints.
-Yes, Roman Catholics to confess their sins to a priest who forgives their sins in the name of the Father, and of the Son, and of the Holy Spirit. The priest is a human just like you and me and will fall in sin just like you and me.
-Holy water is blessed in the name of the Holy Trinity. Just plain water cannot wash away the spiritual scares sin leaves behind but if it’s blessed in the name of the Father, and of the Son, and of the Holy Spirit, only then does it preform such actions.(At least so I think.) Holy Water next to the door is supposed to remind us of our baptism when making the sign of the Cross.