First principle of Scriptural interpretation. You interpret Scripture by comparing it to itself. It is its own interpreter, dictionary and commentary. Thayer's Greek Lexicon is wrong. "Paradise" is not the "top part of Hades"! We know this because the Scriptures themselves tell us what "paradise" is.
So, let's deal with the word "paradise" first.
"Paradise" is Strong's # 3857.
Sheol is Stong's # 86 (which is Greek word Hades) "Sheol" is Hebrew word for the Greek word "Hades". "Sheol" itself is not used in the New Testament. There are Greek transliterations of Hebrew words in the New Testament, but this is not one of them.
The Greek word "Paradise" used here is borrowed from the Persian language. It means "garden" or "park".
Revelation 2:7 tells us "the tree of life" is in "the paradise of God".
The second place this word is used is 2 Corinthians 12:4. In this passage Paul is relaying something he heard from a man who was "caught up to paradise".
Now verse 2 of this same passage tells us where the "paradise of God" is. It's in the "third heaven".
So when Jesus told this thief "Today" (He did not say Sunday morning. He said TODAY) you will be with me in paradise; He is NOT talking about Sheol! We know from the rest of Scripture that Jesus is telling this thief he will be in the 3rd heaven, in paradise where the tree of life is. And since it seems highly likely the thief died after Jesus did; the thief never went to Sheol. Because for the believer: "to be absent from the body is to be present with the Lord". (2 Corinthians 5:6-8)
Next - We'll deal with "Sheol". When did Jesus go to Sheol?
Psalm 16:10
Acts 2:27
Acts 2:31
"You have not left my (his) soul in hell".
Because Acts quotes Psalms. This is how we know "hades" and "Sheol" are the same place.
Next passage (you quoted this one already)
1 Peter 3:18-19
18 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
19 By which also he went and preached unto the spirits in prison;
Next passage (you quoted this one too)
Ephesians 4:9-10
9 (Now that he ascended, what is it but that he also descended first into the lower parts of the earth?
10 He that descended is the same also that ascended up far above all heavens, that he might fill all things.)
Now compare Ephesians 4:9-10 to Matthew 12:40. (Matthew 12:40 refers back to Jonah 1:17) This is the "3 days and 3 nights in the heart of the earth" passage.
So, comparing all these passages together; somewhere in "3 days and 3 nights in the heart of the earth" Jesus's soul goes to Sheol. When did that happen? The answer is in:
Now I'm giving you fair warning here - it's not likely you have ever heard this explanation any place else; but it has totally come out of comparing Bible verses to each other.
Genesis 2:7
Exodus 12
Pslam 104:29
Jeremiah 25:15-16
Jeremiah 30
Daniel 9:27
Jonah 2:2
Matthew 24:6-13
Matthew 24:22&29, Mark 13:20&24
Luke 13:32
Luke 22:43
Luke 23:45
John 12:1-3
John 12:29&31
John 18:1&6
Revelation 20:1-3
John 12:1-3 - Mary (Lazarus's sister) anoints Jesus's feet "6 days before the Passover". Jesus says to Judas: "Leave her alone, against the day of my burial she has kept this." That "time clue" is important; keep it in the back of your head!
Daniel 9:27 "confirm the covenant for a week". From the time Mary anoints Jesus's feet to the day of the crucifixion is "one week". This happens Thursday after sundown (which would have commenced Friday. This is the start of "the great tribulation".
Friday - I don't think the Scriptures tell us specifically what Jesus did Friday day. (Probably preached in the temple.)
Saturday - Sabbath.
Sunday - Triumphant entry into Jerusalem.
Monday - Preached in the temple. Jesus tells a pharisee who comes to Him to warn Him that Herod wants to kill Him. Jesus says "Tell that fox: Behold, I cast out devils, and I do cures today and tomorrow, and the third day I shall be perfected." The "third day" is the commencement of Wednesday.
The word "perfected" here is kind of a weird word. It literally means "to come to the fullness of extension" and is usually used in context of conceiving children. We'd use the world "climax". So Jesus "comes to His climax" which in context of this verse he's referring to casting out devils, doing cures and preaching.
Again though, go back to Daniel 9:27. "the middle of the week" is "Messiah cut off".
Tuesday - Preaches in the temple. Leaves the temple Tuesday before sundown. Jesus and disciples are having a conversation about the temple being destroyed.
Thunder is heard (John 12:29). Jesus explains to them what that thunder is. (John 12:31) Satan has been cast out of heaven. (Revelation 20:1-3)
From here Jesus goes to Bethany to dinner at the house of Simon the leper. (Since we are after sundown - we are now into Wednesday) While He's eating an anonymous woman pours a jar of oil over His head. Again He tells the disciples: "She's done this for my burial". (Matthew 24:6-13)
This is the commencement of the "3 days and 3 nights in the heart of the earth". We know this because exactly 3 / 24 hour periods later - Jesus is dead!
This is also the commencement of the "middle of the weeK" when the Messiah is cut off.
Wednesday - That night Jesus washes the disciples feet and spends much of the rest of the night explaining to them what will happen to Him.
Thursday - They prepare for the passover Thursday morning. Eat it in the upper room Thursday night. They leave the building and go to the "garden" near a brook Cedron; (John 18:1) which is in the Mt. of Olives but is not "Gethsemane".
This is the first attempt to arrest Jesus. A small band come and ask for Jesus. He says. "I am" and they all fall over backwards. (John 18:6) This is one of 4 places in the Scripture that speak of people "falling backwards" and in all those places, they are under the condemnation of God. This should tell you something about the modern "pentecostal" practice of "slain in the spirit". All the people in Scripture who "fell before God" went face down, not backwards. I digress here - but anyways.
They go from this garden to gethsemane. A "gethsemane" is not a garden. There were multiple "gethsemane(s)" in the Mt. of Olives. a gethsemane is where an olive press is. They were located inside caves and this is where they pressed the olives for the oil that burned the lamps in the temple. During feasts when lots of people were in Jerusalem, people would stay in these caves as shelter.
Passover:
So Jesus is in this cave and an angel comes to Him. (Luke 22:43) What angel is this? The passage in English says "....angel from heaven; strengthening him..." but the Greek actually means "display of force against". So, go back to Exodus 12. What angel passes through the land at midnight on the passover to kill the first born? (The angel of death.) So who is this angel who comes to Jesus? (It's the angel of death.) What does the angel of death do to Jesus.
Scripture explains to us that when organisms die, the "breath of life" returns to God. (Psalm 104:29) The breath of life is what makes men "living souls". (Genesis 2:7)
So the angel of death removes the breath of life from Jesus which confines His soul to Sheol. Jesus does not die though; why not? Because He has a Divine nature inseparably joined to a human nature.
This is the "shortening" of "the tribulation" because if "the tribulation is not shortened; no flesh would be saved"! (Matthew 24:22) When is "flesh saved". That has to do with the atonement!
What is "3 days and 3 nights in the heart of the earth". That is speaking of the wrath of God. "Out of the belly of hell you heard my cry." (Jonah 2:2)
Look at Jesus's behavior from the point He leaves the temple to the night of passover. He's extremely distressed. Why? Jeremiah 30 answers that question. This passage also talks about "Jacob's trouble" which is "the great tribulation". What is "Jacob" afraid of? (vs 10) He's afraid of failing God. He's afraid of losing what he came to accomplish. "Jacob" is not eternally lost though on account of Jacob's own righteousness. God being just though, can not rightfully condemn someone who is not a sinner. This is why Jesus's own personal integrity is so important to the atonement.
Jeremiah 25 talks about "the cup of God's wrath" causes the nations to "go mad". "Jacob" is probably also afraid of going insane and that is what will happen if God does not intervene. And this is why the angel of death confines Jesus's soul to hades. Jesus is determined to compete this course of action; but He can't do so if He's an emotional mess.
Now would the Romans have crucified someone they deemed to be insane? Probably not.
Note Jesus's interactions with people from the point this angel leaves until Jesus dies. He remains morally and cognitively intact; but absolutely emotionless.
By the point of "Why have you forsaken me". "I thirst" and "It is finished" Jesus becomes more and more difficult to understand. Those who wrote the Scripture give us by interoperation from the Holy Spirit what Jesus said; but those who were present could not understand Him. The Greek denotes that His "cries" sound like the screeching of a crow. Obviously as His body breaks down; He's no longer able to produce intelligible speech.
Last thing says He bows His head and "gives up" or "sends off" His spirit. This is his human spirit, not the Holy Ghost. The Holy Ghost, just as the Father had already forsaken Him. Jesus dies because the Divine nature is rent from the human nature. This is what actually kills him.
His soul is released from Sheol (as well as the souls of all those He's atoned for who died on the OT side of the cross). Soul and spirt ascend to heaven to stand before the Father as "the lamb that was slain". We see this in Revelation 5. Note who's there with Him. "those who've come out of great tribulation". They are "souls" (not bodies) who "live and reign with Him 1000 years". (Revelation 6:9, Revelation 20:4)
Jesus's body rests on the Sabbath.
Rises from the dead on Sunday before dawn. Ephesians 1:18-22 tell us that Christ begins to "reign" when He rises from the dead.
Now when does Jesus ascend back to the Father? (I.E. in bodily form as a whole person) That happens 40 days later.
Why does Jesus tell Mary not to touch Him? Other people touch Him later on (women included). He says to her that He still has to ascend to the Father; yet we have no other Scriptural evidence that suggests Jesus left the earth and went back to heaven in bodily form between Sunday resurrection and the ascension.
Matter of fact this word "ascend" Strong's # 305. Is only used once in the context of "ascend to heaven". Acts 2:34 talks about David has not "ascended" to heaven before "the Lord says to my Lord sit at my right hand...." When does Jesus "sit down at the right hand of God the Father". That one I'd have to research some more.
This response is long enough right now though, so I leave it as is.