There is a big difference in breaking fellowship with God and losing one’s salvation. It’s the old man (carnal) nature that sins, not the new man created in righteousness and holiness that commits sin. The old man is dead, crucified with Christ. He just hasn’t accepted his death yet.
Your first sentence again fails to interact with the texts at issue, which are neither addressed to unbelievers nor simply warning of loss of fellowship.
As for the the flesh committing sin versus the new man, that is a distinction without a difference as concerns saving faith versus denying it. It is the heart where sin begins, (Mk. 7:21-23) and it is the error of certain gnostic antinomians to hold one may sin with his flesh without affecting his relationship with God. We are not to repent for having a sinful nature, but for yielding to it, and to wilfully live after the flesh, (Gal. 5:19-21) or to impenitently continue in a something which is manifestly sin and could repent from, (1Tim. 5:8) or to assent to a false gospel, (Gal. 5:1-4) is to deny the faith, and its benefits.