1.) Your Act of Contrition can be as simple as, “Be merciful to me, O Lord, a sinner.” Once in a while, because I go to confession EARLY in the morning usually, I mentally fuzz up. Sometimes, I’m so overcome by this gift of forgiveness, a standard formatted prayer does not seem worthy of the gift I’m being given. I wing it. I still get absolved.
2.) Well, what if you went to your friend’s house and when he or she opened the door, pushed aside him or her, flopped in a chair, and didn’t even say hello? The Lord God holds open house and is about to Give Himself to us, and it would be rude to ignore Him by not even acknowledging His Presence- esp. if the Tabernacle is up front and center. So, tell Him hi, how are You, maybe even mention a little bit about your week or what have you. Do you have to do it? No. But it would seem the polite thing to do.
3.) You are supposed to talk with Your Guest, the One who has just entered your body with His. How you do that is up to you.
4.) Mass ends when the priest says, “The Mass is ended, go in peace,” and the people reply, “Thanks be to God.” Again, polite people, esp. in the U.S., wait until the priest has either gone back to the sacristy, or made it to the narthex to shake hands. I have heard of other countries, and some people in the U.S., who “bag” after “Thanks be to God” and are on the sidewalk or in the parking lot before the priest can even get lined up to leave.