RE: This day you shall be with me in Paradise
When a man dies he doesn't go to prison, he goes to his grave, often translated hell. Hell is just a hole in a ground.
"By the sweat of your face You will eat bread, Till you return to the ground, Because from it you were taken; For you are dust, And to dust you shall return."
When an angel dies where does he go? I'm not sure they can die.
1 Peter 3:19 can easily be rendered "By which also he went and preached unto the spirits of Watchers"
Watchers were the sons of God who lusted after the daughters of men in Genesis 6. God judged them and bound them in hell in chains of darkness until the day of Judgement. 2 Peter 2:4, Jude 6.
So, I'm guessing here that while Jesus was dead he traveled in Spirit underground to the place where those angels were kept. He preached to them about his birth and sacrifice and offered them salvation.
Your assumption I believe is correct that there is a difference between "spirit world" and "heaven". Heaven is a geographical region, a location (just look up and you see it). Your use of the term "spirit world" is somewhat misleading. It is this world in which we live but the part of it that most men can't see. Check out 2 Kings 6:15-17:
"15 And when the servant of the man of God was risen early, and gone forth, behold, an host compassed the city both with horses and chariots. And his servant said unto him, Alas, my master! how shall we do?
16 And he answered, Fear not: for they that be with us are more than they that be with them.
17 And Elisha prayed, and said, Lord, I pray thee, open his eyes, that he may see. And the Lord opened the eyes of the young man; and he saw: and, behold, the mountain was full of horses and chariots of fire round about Elisha."
the spirit world is around us, it is in a dimension that we mortals cannot see. However angels can be sent to earth, there are numerous accounts. We also have the evidence from the Garden of Eden that God did visit Adam and Eve. Just because a person is seen in one location it does not mean it is their usual residence. Certainly the physical body goes into the ground if they are buried. The spirit is what animates the physical body. There is a great deal of misunderstanding who the sons of God were. It is mistakenly assumed to be beings similar to those found in Greek mythology who would come to earth and have sexual relations with mortal women. the term sons of God refers to mortal men who made covenants with God, do the scriptures not repeatedly refer to us as children of God? This message is consistently found throughout scripture. God is the father of our spirits, how did Jesus teach us to pray? how did he address God? Our FATHER who art in heaven. When the Jews accused him of blasphemy what did he say to them? Ye are all gods. The key difference between Jesus Christ and us is that God the Father is the father together with Mary of his earthly body, hence he is called the Only Begotten of the Father. The message of the scriptures is a simple and consistent path.