Steve,
You wrote:
On the Day of Pentecost, when Peter had made no allusion to a final, or post-mortem, judgment (he only said that all Christ's enemies would be made His footstool—that is, would be brought into subjection to Him), the people were pricked to the heart. They seemed frightened when they came and asked, "What must we do?" This would seem to be a reasonable response to that information.
If it is as you appear to say, that the fright was due to their being convinced that Jesus was now their Lord, I do not see what is so terrifying about that, absent any knowledge of consequences of failure to yield to His lordship. After all, probably most all of them recognized the Lordship of Yahweh; all that was needed would be to recognize Yahweh's delegation of authority to Jesus, just as we do. But I am unsure of what you mean by subjugation, which strongly implies forceful conversion. Come to think of it, that sounds like the universalist's picture of hell.
But I think the footstool allusion is much more than being put under someone's authority or subjugation. There is very good evidence that it is a reference to being destroyed, as in executed. Perhaps you have seen pictures of ancient art depicting a conquering ruler with his foot on the neck of his prostrate opponent just before plunging the spear into his victim. The "being made a footstool" references in the NT obviously come from Psalm 110, concerning which numerous commentators refer to Joshua 10:22-26:
Joshua 10:22-26
New King James Version (NKJV)
22. Then Joshua said, “Open the mouth of the cave, and bring out those five kings to me from the cave.” 23. And they did so, and brought out those five kings to him from the cave: the king of Jerusalem, the king of Hebron, the king of Jarmuth, the king of Lachish, and the king of Eglon.
24. So it was, when they brought out those kings to Joshua, that Joshua called for all the men of Israel, and said to the captains of the men of war who went with him, “Come near, put your feet on the necks of these kings.” And they drew near and put their feet on their necks. 25. Then Joshua said to them, “Do not be afraid, nor be dismayed; be strong and of good courage, for thus the Lord will do to all your enemies against whom you fight.” 26. And afterward Joshua struck them and killed them, and hanged them on five trees; and they were hanging on the trees until evening.
Psalm 110 seems to speak of much the same fate for God's enemies. Viewed in this light, the symbolism of the foot on the neck, Peter's "footstool" reference would be a terrifying thing indeed.
I must add that I agree it makes no sense to preach hell, or the judgement and its consequences, to an atheist. That would only provoke derision. However, many people we would classify as unbelievers do think that there is life after death an an accounting of some sort to be made. They mostly just think if their good outweighs their bad they have a ticket to heaven. Thus warnings about judgement and its consequences have a place in evangelism, used with good judgement, of course.