New American Standard Bible Psalm 68:18
You have ascended on high, You have led captive [verb H7617] Your captives [noun H7628]; You have received gifts among people, Even among the rebellious as well, that the LORD God may dwell there.
Brenton Septuagint Translation
Thou art gone up on high, thou hast led captivity [G162] captive [G161], thou hast received gifts for man, yea, for they were rebellious, that thou mightest dwell among them.
The double-word idiom means leading in triumph a long train of captives.
English Standard Version
You ascended on high, leading a host of captives in your train and receiving gifts among men, even among the rebellious, that the LORD God may dwell there.
This idiom appears in other verses, Judges 5:
Wake up, wake up, Deborah! Wake up, wake up, break out in song! Arise, Barak! Take captive your captives, son of Abinoam.’
Brenton Septuagint Translation
2 Chronicles 28:
And the Lord his God delivered him into the hand of the king of Syria; and he smote him, and took captive of them a great band of prisoners, and carried him to Damascus. Also God delivered him into the hands of the king of Israel, who smote him with a great slaughter.
Is there a real difference between "captives" [H7628/G161] and "captivity" [H7617/G162]?
Yes, H7628/G161 is a noun and H7617/G162 is a verb. Together, it forms an idiom that means leading in triumph a long train of captives.
Now to the NT, English Standard Version Ephesians 4:8
Therefore it says, “When he ascended on high he led [G162] a host of captives [G161], and he gave gifts to men.”
King James Bible
Wherefore he saith, When he ascended up on high, he led captivity [G162] captive [G161], and gave gifts unto men.
OP: in ESV 161 aichmalosian is captives, but in KJB 161 is captivity. Captives sounds like people, but captivity sounds like an abstraction or principle
KJB translated G161 as "captive" singular, not as "captivity". The original Greek is singular.
Unlike KJB, ESV translators decided to translate the idiom G162 G161 in plural form as "he led [G162] a host of captives [G161]".
KJB took a more literal approach than ESV.
Has the New Testament put a New Testament twist on an Old Testament Scripture, to bring out a New Testament meaning?
Not in terms of the Greek wording.
Ephesian 4:8 uses exactly the same sequence G162 G161 as in LXX's Psalm 68:18.
Now the question becomes:
Does the LXX Psalm 68:18 idiom denote the same concept as that in Ephesian 4:8?
Well, not exactly the same. The concept was updated with the Roman practice of triumphal procession.
He led captivity captive - The meaning of this in the Psalm is, that he triumphed over his foes. The margin is, "a multitude of captives." But this, I think, is not quite the idea. It is language derived from a conqueror, who not only makes captives, but who makes captives of those who were then prisoners, and who conducts them as a part of his triumphal procession. He not only subdues his enemy, but he leads his captives in triumph. The allusion is to the public triumphs of conquerors, especially as celebrated among the Romans, in which captives were led in chains (Tacitus, Ann. xii. 38), and to the custom in such triumphs of distributing presents among the soldiers;
The above picture was the updated Roman meaning. Futher, when it applies to Christ's victory, there is more meaning attached to the idiom:
When Christ ascended to heaven, he triumphed ever all his foes. It was a complete victory over the malice of the great enemy of God, and over those who had sought his life. But he did more. He rescued those who were the captives of Satan, and led them in triumph. Man was held by Satan as a prisoner. His chains were around him. Christ rescued the captive prisoner, and designed to make him a part of his triumphal procession into heaven, that thus the victory might be complete - triumphing not only over the great foe himself, but swelling his procession with the attending hosts of those who "had been" the captives of Satan, now rescued and redeemed.
What is Paul's understanding of Psalm 68?
He understood it as leading in triumph a long train of captives in King's David's sense. When he wrote Ephesian 4:8, however, he understood it in its Roman sense of the idiom and onto Christ's victory.