It means that the crucifixion will not be repeated. It does not have to be. It makes ample provision for repentance and returning to the fold.
Yes. That is correct.
Hebrews 3:
12Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. 13But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. 14For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end; 15While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
It is only to those who harden their hearts that God gives over to a reprobate mind. Men have the choice to come, and men have the choice to go. God does not carry around a bottle of Gorilla Glue.