Good questions,
The Bible does teach that fallen man, is unable to come to a place of repentance because of his depravity. So the fact that man is unable to repent, unless God grants him repentance means that he is totally depraved. Man simply doesn't have the desire to forsake his life of sin, because he loves it too much to willingly give it up.
We could call atonement, atonement instead of limited atonement if Jesus atoned for everyone. But He said He came to atone for those which His Father gave Him. Jesus never said I've come to save everyone, He said He came as a sword to pit son against father and mother against daughter. He came to divide the world.
The elect don't endure to the end by their own power, it is God the Holy Spirit who keeps us from falling away.
1 John 2:19 says,
They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.
This verse shows that the false Christians are exposed because they don't endure, they eventually get sick of pretending. But the true believers are kept on the right path by the Holy Spirit and they never fall away.