What is the spirit? What does this word (spirit) mean in Greek, rather than the meaning Christendom wants to give it as a synonym for soul?
The soul and the spirit are two entirely different words with two entirely different meanings.
Strongs defines "spirit" this way....
"πνεῦμα pneûma, pnyoo'-mah; from G4154; a current of air, i.e. breath (blast) or a breeze; by analogy or figuratively, a spirit, i.e. (human) the rational soul, (by implication) vital principle, mental disposition, etc., or (superhuman) an angel, demon, or (divine) God, Christ's spirit, the Holy Spirit".
So if the primary definition of "spirit" is our "breath"......that which keeps the soul (body) alive, then Stephen was asking Jesus to receive his life as the one charged by God with the resurrection.....giving back the spirit (breath) of life in the same way that he made Adam alive with the "breath (spirit) of life" in the beginning.
Those described in 1 Thess 4:17 are not the great crowd.....they are the elect who are left alive on earth at the coming of the Lord....These will not need to sleep in death as the other members of the elect have from the first century onward. They will be instantly transformed and "caught away to meet the Lord in the air". Read the whole passage....v13-17.
Again this is Paul speaking of the elect collectively....
"One body there is, and one spirit, just as you were called to the one hope of your calling; "
This is the "body of Christ" "called to the one hope" of all going to heaven.....it is their "calling" (Heb 3:1)
Not all have the "heavenly calling".
Paul wrote...
"to the congregation of God that is in Corinth, to you who have been sanctified in union with Christ Jesus, called to be saints, together with all those everywhere who are calling on the name of our Lord Jesus Christ, their Lord and ours." (1 Cor 1:2)
Two groups here....both have Jesus as their "Lord".
The spirits in prison were the demon angels who rebelled in Noah's day....not the spirits of dead people.
2 Peter 2:4...
"Certainly God did not refrain from punishing the angels who sinned, but threw them into Tarʹta·rus, putting them in chains of dense darkness to be reserved for judgment."
Jesus, after his resurrection in the spirit, went to pronounce that judgment message to those wicked spirits in Tartarus, which is not "hell" as many Bibles mistranslate.