The Jews were in the olive tree to begin with because they were the "natural branches" and not because they were all saved. (vs. 19-21) Because of their unbelief and hard hearts God removed His gracious hand from them as a people overall and broke them off from His goodness (but only for a time after which they will be restored - Romans 11:24-26). We Gentiles have now been grafted into God's goodness and are the recipients of His blessings. Paul's warning is that we should not become arrogant because we might lose the goodness and blessings of God just like the Israelites lost the goodness and blessings of God, but this doesn't speak of losing salvation.
Professing Christians who are Gentiles are corporately in outward covenant with Christ so, it would appear that Romans 11 is speaking about the question of collective ecclesiology and not individual soteriology. I see the warning to this collective body, which is corporately joined to Christ and is in a covenant relationship, but how could this mean that every individual in it is in saving union with Christ? Hence the "cut off." Union with Christ applies to the elect, and only for the elect are, "the gifts and the call of God are irrevocable." (verse 29) But since non-elect covenant members are mixed in, Christ clearly appears to have non-elect branches and while they may be joined outwardly in covenant with Christ, since they have professed faith in Jesus, the faith of some of them is spurious.