I always enjoy helping those that are open to help.
One thing that we know about verse 21 is that it cannot be warning the body of Christ. We are sealed with the Holy Spirit upon entry into the body and nothing can remove us.
Rom 8:1 (AKJV/PCE)
(8:1) [There is] therefore now no condemnation to them which are in Christ Jesus, who walk not after the flesh, but after the Spirit.
Rom 8:38-39 (AKJV/PCE)
(8:38) For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, (8:39) Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord.
The ones grafted in are NOT the body of Christ, as the body of Christ is a NEW CREATURE.
So all of the talk in chapter 11 is about gentiles with Israel before the present dispensation.