You're exactly correct. People who believe a person can lose their salvation also have problems with Heb 6:4-6 simply for the reason you've stated-if they were to have an initial salvation and fell away, what's the point? They can never be brought back to salvation.
Hebrews is an evangelistic call to the Hebrews and should be read that way. Consequently when one gets to Heb 6 the writer states to the Jews; "...leaving the elemental teachings about the Christ...", he makes the points that
2) the Jews have tasted of the heavenly gifts (through the history of the Hebrew nation),
3) the Jews have been made partakers of the Holy Spirit (through the covenant actions of God throughout their history), and
4) the Jews have tasted of the good works of God.
Hebrews 6 is nothing more than an outward calling of God to the Hebrews.
1 Wherefore leaving the word of the beginning of Christ, let us go on to things more perfect, not laying again the foundation of penance from dead works, and of faith towards God, 2 Of the doctrine of baptisms, and imposition of hands, and of the resurrection of the dead, and of eternal judgment. 3 And this will we do, if God permit.
Her St. Paul is saying that the recipients of the letter are prepared to leave the rudimentary Christian doctrine for advanced learning upon which they now embark. It should be clear from this preamble that it is addressing baptized Christians.
4 For it is impossible for those who were once illuminated, have tasted also the heavenly gift, and were made partakers of the Holy Ghost, 5 Have moreover tasted the good word of God, and the powers of the world to come,
Stop the tape. Who are these "those", again? They were illuminated by the very things Paul enumerates as the beginner's doctrine, faith, penance, baptism, imposition of hands (conferring in Christian practice the gift of the Holy Ghost).
6 And are fallen away: to be renewed again to penance, crucifying again to themselves the Son of God, and making him a mockery.
Indeed, there is no second baptism after apostacy. Sin crucifies Christ anew.
7 For the earth that drinketh in the rain which cometh often upon it, and bringeth forth herbs meet for them by whom it is tilled, receiveth blessing from God. 8 But that which bringeth forth thorns and briers, is reprobate, and very near unto a curse, whose end is to be burnt.
Note that the blessing is accomplished while the curse is "very near". This gives hope and moderates the harsh "it is impossible" of verse 4.
9 But, my dearly beloved, we trust better things of you, and nearer to salvation; though we speak thus. 10 For God is not unjust, that he should forget your work, and the love which you have shewn in his name, you who have ministered, and do minister to the saints.
Now another group of Christians is described, who perseveres to the end. They are not saved, but "nearer to salvation". The danger of falling away is present in them; but there is hope in the mercy of God Who will not forget their good work.
There is absolutely nothing in this glorious passage that contradicts the Catholic theology. It is in fact Catholic theology: superabundance of grace, perseverance in faith through works, hope of salvation, and danger of losing the gift of baptism.
I see a problem in definition again. "Salvation" doesn't mean "eternal vision of God in heaven" in EVERY case in Scripture. The Bible uses words that have different meanings. Faith has different meanings. So does Salvation. There is "temporal" salvation and "eternal" salvation. The confusion you are having is thinking that a person who has been healed from sin can NEVER become sick by it again...Weren't the Jews "saved" from the slavery of Egypt, only to "die" in the desert? The Church Fathers and St. Paul in 1 Corinthians both see this as our spiritual example - that the "saved" can still become sick and die spiritually.
It is impossible to restore them (the Jews) to repentance because they reject the entire history that God has given them.
Wrong. First, Christ said that nothing is impossible for God. Secondly, perhaps you should read Chapter 11 of the Letter of Paul to the Romans...
Even so then, at this present time there is a remnant according to the election of grace (v. 5)
I say then, have they stumbled that they should fall? Certainly not! But through their fall, to provoke them to jealousy, salvation has come to the Gentiles. Now if their fall is riches for the world, and their failure riches for the Gentiles, how much more their fullness! (v. 11-12)
and for the kicker...
Therefore consider the goodness and severity of God: on those who fell, severity; but toward you, goodness, if you continue in His goodness. Otherwise you also will be cut off. And they also, if they do not continue in unbelief, will be grafted in, for God is able to graft them in again. For if you were cut out of the olive tree which is wild by nature, and were grafted contrary to nature into a cultivated olive tree, how much more will these, who are natural branches, be grafted into their own olive tree? (v. 22-24)
It's clear that you are mistaken on your interpretation of Hebrews and the idea that God cannot graft people back onto the "olive tree".
Regards