Luke wrote “we” because he went with Paul on the ship. Luke was writing about things that he saw. Some scholars think Luke and Aristarchus were Paul’s servants on the ship. Other scholars think “we” included everyone on the ship.
Paul reminded the sailors they did not listen to him because he wanted them to know he spoke wisely when he first spoke to them. He was hoping they would listen to the things he said now. That is, he was going to give them wise advice once again.
Paul talked about salvation from the storm and shipwreck. He was not talking about the forgiveness of sins. Paul wanted them to eat so they will have the strength to survive.
See: Save (Salvation, Saved from Sins); Forgive (Forgiveness, Pardon); Sin
The soldiers wanted to kill the prisoners to keep them from escaping because Roman leaders killed Roman soldiers when a prisoner escaped (see: Acts 12:18-19; 16:27). However, God wanted to bring Paul to Rome. The Roman soldier did God’s will when he stopped the soldiers from killing the prisoners.