Problem once again with using this passage from Hebrews is that it is saying those who refuse to repent and come back to the Lord are putting Him to an open shame for themselves. Not that it is impossible to fall away and not end up with salvation. Read verse 6 real closely;
[SUP]6[/SUP]
If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
They are doing the shame to themselves by refusing to repent of their ways, as they believe one of two things; either they don't have to repent or that they love darkness more then light. You are right on the first half though that true believers do not have to worry about repenting again and the laying on of hands as they will be walking properly in the faith, and a one time baptism is all one needs.
But the bible makes it clear by Jesus, Holy Spirit, Paul, and James that many will fall away from the faith and they will not end up with eternal life unless they come back to the Lord.
Matthew 24:10-13
And then
many will fall away and betray one another and hate one another. And many false prophets will arise and lead many astray. And because lawlessness will be increased, the love of many will grow cold. But the one who endures to the end will be saved.
1 Timothy 4:1
But the Spirit explicitly says that in later times some will fall away from the faith, paying attention to deceitful spirits and doctrines of demons,
Colossians 1:22-23
In the body of His flesh through death, to present you holy, and blameless, and above reproach in His sight— if indeed you continue in the faith, grounded and steadfast, and are not moved away from the hope of the gospel which you heard, which was preached to every creature under heaven, of which I, Paul, became a minister.
James 5:19-20
19 Brethren, if anyone among you wanders from the truth, and someone turns him back, 20 let him know that he who turns a sinner from the error of his way will save a soul from death and cover a multitude of sins.