If a person consecrates and attempts the narrow path to heaven as a new creature rather than the wide path/highway to live in "heaven on earth," then later realizes he is as human and sinful as the next person and decides to change his goal, will that person enter second death? And if so, where is the grace in that?

Our decision to follow in Jesus’ footsteps is a very serious one. This is why Jesus said to “count the cost.” Luke 14:27-30,33 (NKJV), “And whoever does not bear his cross and come after me cannot be my disciple. For which of you, intending to build a tower, does not sit down first and count the cost, whether he has enough to finish it lest, after he has laid the foundation, and is not able to finish…So likewise, whoever of you does not forsake all that he has cannot be my disciple.”

The ransoming blood of Jesus can only be applied to a person once. Hebrews 6:4-6 (NKJV), “For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put him to an open shame.” 

However, many, many people take the first step of believing that Jesus died for them, but they do not go on to sacrificing their earthly lives. That’s why the Apostle Paul wrote, “I beseech you therefore, brethren (believers in Jesus), by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service” Romans 12:1. 

These dear ones in your example have not received the Holy Spirit. They try to live righteously according to God’s will but are not begotten as “new creatures.” And of course they find they are unable to walk the narrow way. It is only by God’s power and grace that we can, indeed, deny ourselves.

We assume in this case they will receive an earthly resurrection – by God’s grace. Life is a gift from God.