I wasn't trying to force anyone's hand with that, just thought it relevant.
As for the sign of Jonah, did you notice that it predicts the fall of Jerusalem? Nineveh was spared, Jerusalem was not. A year for a day, for each day of Jonah's walk.
But as for spirits in prison, humans aren't spirits. Spirits are spirits. We are told of spirits which were once disobedient, they were rebellious in the days of Noah, they are kept chained until judgment. Perhaps confusion might arise because people today often think of a church when they hear the word Preach... But the word is closer to "proclaimed."
1 Peter 3:19-20 KJV
By which also he went and preached unto the spirits in prison; [20] Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
2 Peter 2:4-5 KJV
For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment; [5] And spared not the old world, but saved Noah the eighth person , a preacher of righteousness, bringing in the flood upon the world of the ungodly;
As for context see Genesis 6
Genesis 6:2-4 KJV
That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose. [3] And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. [4] There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Other references to Sons of God in the Old Testament also seem to refer to spirits (not men)
Job 2:1 KJV
Again there was a day when the sons of God came to present themselves before the Lord , and Satan came also among them to present himself before the Lord.
Job 38:4-7 KJV
Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding. [5] Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it? [6] Whereupon are the foundations thereof fastened? or who laid the corner stone thereof; [7] When the morning stars sang together, and all the sons of God shouted for joy?
Just saying that spirits are spirits and men are men. There may be a spirit in man that he requires to live, but we aren't told that it functions by itself. Angels are created spirits and that seems to fit Peter's context without conflict, even in reference to Noah.