It's impossible that Paul was including the gift of eternal life in Romans 11:29 because that directly contradicts what Paul and John say here that one has to continue in the word by which they first believed in order to be saved and have eternal life:
"1Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, 2by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain." (1 Corinthians 15:1-2 NASB)
" 23Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also. 24As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father.
25This is the promise which He Himself made to us: eternal life.
26These things I have written to you concerning those who are trying to deceive you."
9Anyone who goes too far and does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he has both the Father and the Son.
11 ...God has given us eternal life, and this life is in His Son. 12He who has the Son has the life; he who does not have the Son of God does not have the life.
(1 John 2:23-26 NASB, 2 John 1:9 NASB, 1 John 5:11-12 NASB)
Besides, simply looking at Romans 11 shows us what Paul means by the gifts and calling to Israel being irrevocable:
28From the standpoint of the gospel they (Israel--see context) are enemies for your sake, but from the standpoint of God’s choice they are beloved for the sake of the fathers; 29for the gifts and the calling of God are irrevocable. 30For just as you once were disobedient to God, but now have been shown mercy because of their disobedience, 31so these also now have been disobedient, that because of the mercy shown to you they also may now be shown mercy." (Romans 11:28-31 NASB)
Instead, it shows that one's interpretation of all these other verses doesn't line up with what Paul clearly stated in Romans. There is absolutely nothing in either the entire text of Romans, or the local context, to show that Paul had something other than what he had already described as gifts of God as being irrevocable.
If Paul had not meant to include the gifts that he had already described as God's gifts, then he would have had to clearly make a statement to that effect.
It would have been unconscionable for Paul to have described several things as gifts from God and then make a statement about God's gifts being irrevocable yet not mean exactly those specific gifts and not mean them specifically.
And none of this changes the absolute fact that justification and eternal life were described as gifts of God by Paul and Paul never excluded them from what he meant in Rom 11:29.Israel rejected Christ, and the gospel went to the gentiles as a result. But because the promise of God's gifts and calling to Israel is irrevocable, Israelites who don't reject Christ will be shown mercy (vs. 31) and will inherit the irrevocable promises made to Abraham's natural descendants. A simple read of the passage shows us that's exactly what he's saying. It's hardly a passage to somehow prove that a person can believe and then not believe but stilled be saved because Paul says the gifts and calling of God to Israel are irrevocable. Even in this passage, Paul points out the necessity for the Israelites to have faith to be connected back to the root:
"if they do not continue in their unbelief, will be grafted in, for God is able to graft them in again." (Romans 11:23 NASB).