Don't want to hijack the thread but I feel I should answer a few of these questions....
Papal infallability:
Matt 16:18-19 (NAB)I also say to you that you are Peter, and upon this rock I will build My church; and the gates of Hades will not overpower it. 19 “I will give you the keys of the kingdom of heaven; and whatever you bind on earth shall have been bound in heaven, and whatever you loose on earth shall have been loosed in heaven.”
You will also note when ever there is a dispute in acts... everybody gets a say, but when Peter speaks up it is considered decided. Also Papal infalitiblity was a major cause of the great east/west schism. So there was at least some understanding of this over a thousand years ago.
Infant Baptism.
Acts 16:15 Acts 16:33 1 Cor. 1:16 all speak of baptising entire housholds. No mention of not baptising children. We baptise babies becasue baptism takes the place of circumcision and babies where circumcised.
Baptism by sprinking
The word for baptism in biblical text is used for emersion AND sprinkling. The bible never tells us specificly how to baptise. We are just told to do it in Jesus name, or in the name of the Father Son and Holy Spirit. Also the new testiment speaks of baptising thousands at a time in the Jordan river, that river is just a creek it would have been very challenging to do all of them with immersion. Also it is unlikely that you would dunk infants as a form of baptism. Catholics believe in full imersion baptism as well, when our Church is done being renovated, we will have a baptismal font to allow for immersion baptism.
"Sign of the Cross"
That is just a prayer that many Catholics do. It is not a doctrinal thing. Don't all believers say prayers that arn't directly found in scripture?
Eucharist
As to when and how we recieve communion (The cup and/or the bread) that is a practice or tradtion of the Church. Again nothing doctrinal.
Marriage to non Catholics
I don't know enough about this one to comment sorry.
Immaculate Conception of Mary
Luke 1:28 (ASV)And he came in unto her, and said, Hail, thou that art highly favored, the Lord is with thee.
Sometimes it is rendered as "Hail Mary, Full of Grace" both translations are not very good here. The word used is Kecharitomene, it is not used as an adjective instead it is a title given to Mary. The title means complete or perfected in grace. A much better reading of this text would be "Hail, full and complete in grace, the Lord is with you..." We just add Mary in there to make it read better in our language. How could Mary already be full of grace before Jesus was even born and probably before he was even conceived?