The word "Israel" is being used as a parable.
Romans 9:4
the people of
Israel. Theirs is the adoption to sonship; theirs the divine glory, the covenants, the receiving of the law, the temple worship and the promises.
Romans 9:6
It is not as though God’s word had failed.
For not all who are descended from Israel are Israel.
Romans 11:25
I do not want you to be ignorant of this mystery, brothers and sisters, so that you may not be conceited:
Israel has experienced a hardening in part until the full number of the Gentiles has come in,
Even when I agree "Israel is being used as a parable" in Romans 9:3-8, it sets up the Gentiles (who have the promise of God's salvation in Jesus because through Abraham the earth is blessed) as being part of the "so all Israel is saved" without contradicting some of "Israel" has died "enemies of the gospel" yet in death are "shown mercy" because the gifts and calling of God are irrevocable :
6 But it is not that the word of God has taken no effect. For they are not all Israel who are of Israel,
7 nor are they all children because they are the seed of Abraham; but, "In Isaac your seed shall be called."
8 That is, those who are the children of the flesh, these are not the children of God; but the children of the promise are counted as the seed. (Rom. 9:6-8 NKJ),,,
6 But it is not that the word of God has taken no effect. For they are not all Israel who are of Israel,
7 nor are they all children because they are the seed of Abraham; but, "In Isaac your seed shall be called."
8 That is, those who are the children of the flesh, these are not the children of God; but the children of the promise are counted as the seed. (Rom. 11:17 NKJ)...
23 And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again.
24 For if you were cut out of the olive tree which is wild by nature, and were grafted contrary to nature into a cultivated olive tree, how much more will these, who are natural branches, be grafted into their own olive tree?
25 For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has come in.
(Rom. 11:23-25 NKJ)
The "fullness of the Gentiles" hasn't "come in" (to the Israel of God) until the End Time, implying some "children of the flesh" who are "children of God" "do not continue in unbelief" after physical death. "All Israel" includes all since the beginning of Time, to the end of time.
26
And so all Israel will be saved, as it is written: "The Deliverer will come out of Zion, And He will turn away ungodliness from Jacob;
27 For this is My covenant with them, When I take away their sins."
28
Concerning the gospel they are enemies for your sake,
but concerning the election they are beloved for the sake of the fathers.
29
For the gifts and the calling of God are irrevocable.
30 For as you were once disobedient to God, yet have now obtained mercy through their disobedience,
31 even so these also have now been disobedient, that through the mercy shown you they also may obtain mercy.
32
For God has committed them all to disobedience, that He might have mercy on all.
(Rom. 11:26-32 NKJ)
Therefore, even when your premises are accepted, universal postmortem opportunity for "all Israel" is taught in this context as it requires "some of those who died enemies of the gospel" will be shown Mercy at Christ's Second coming, when the deliverer comes "out of Zion". 
