They were given before the incarnation. Existing entities??? Where do you see that in scripture?
There's actually quite a bit of Scripture that makes it clear that, while the Lord certainly knew who would be saved, they were not saved (redeemed, in an eternal context) until Christ redeemed them from their sins.
You are suggesting a salvation that did not exist prior to the Cross.
Here are a couple verses to consider:
Hebrews 9:12-15
King James Version
12 Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.
13 For if the blood of bulls and of goats, and the ashes of an heifer sprinkling the unclean, sanctifieth to the purifying of the flesh:
14 How much more shall the blood of Christ, who through the eternal Spirit offered himself without spot to God, purge your conscience from dead works to serve the living God?
15 And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
If Christ entered into "the Holy Place" through His blood, which took place after the Incarnation, what were these people you speak of saved by? And, are you suggesting that people under the Law could be saved by the Covenant of Law? Because, right here, it is making the point that those under the Law were still in need of redemption when Christ offered Himself up for them.
Notice:
Hebrews 9:24
King James Version
24 For Christ is not entered into the holy places made with hands, which are the figures of the true; but into heaven itself, now to appear in the presence of God for us:
It is significant that Christ entered into Heaven, right?
Also notice:
Hebrews 10:16-20
King James Version
16 This is the covenant that I will make with them
after those days, saith the Lord, I will put my laws into their hearts, and in their minds will I write them;
17 And their sins and iniquities will I remember no more.
18 Now where remission of these is, there is no more offering for sin.
19
Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus,
20
By a new and living way, which he hath consecrated for us,
through the veil, that is to say, his flesh;
The Old Testament Holy of holies was forbidden to men apart from the High Priest and stood as a figure for the reality of entrance to Heaven. Here, we see that entrance into the True Holiest of All is accomplished by one thing: Christ's death in the stead of the sinner.
We can set an approximate date in history when Heaven became available to those who were not just saved by grace through faith but were also eternally redeemed through the death of Christ. That is, His dying in the stead of the sinner.
So, yes, they did sit in Hades and await their redemption. No man entered into the Holiest of All until their entrance was made possible by Christ.
God bless.