Some ‘key’ parts to the statement in Hebrews. Let’s looks at these. First, ….“then have fallen away”. The question to ask is fallen away from what?
The first word of the first verse in Hebrews chapter 6 is ‘therefore’. So we need to find out why that’s there for. Chapter 5 is part of Paul’s (?) outline explaining the way the Mosaic covenant dealt with ‘sin, and then the difference the cross made to this.
Now, once they understood this, (were ‘enlightened’), that is, that through the cross that ‘sin’ had been dealt with *once and for all’, (Romans 6:10). But then if you go back to seeking forgiveness for your ‘sin’ via the blood of animals, you are cancelling out, ignoring what you’ve learnt about what Jesus accomplished. It’s like the cross didn’t happen. And if you ‘act’ as if the cross didn’t make a difference, then it’s impossible - “to be restored to repentance”, and if you try, it’s as if “they themselves are crucifying” Jesus again and again, in fact, each time they seek repentance.
The point is that this is not talking about salvation, it’s talking about repentance. As to what part ‘repentance’ plays In salvation is another question, but nevertheless this passage is talking about the how of repentance from sin.
The second passage you quoted (1 Corinthians 5) clearly shows that salvation is not lost via ‘sin’. That man ‘sinned’. All ‘sin’ is in the flesh. He (at that stage) refused to repent, but was ‘covered’ or ‘protected’ by Grace, under the covering of the church. So Paul exercised his authority (because the ‘church leaders’ didn’t) and removed that ‘protection’. Exposing the man’s ‘sin’ to judgement. In the intent that this would lead to repentance. (Which it did, in Paul’s follow up letter he thanked the church for restoring the man back into fellowship.)
But the ‘key’ here is … “*he himself will be saved on the day the Lord returns.”. That is, despite his ‘sin’ he was saved, but nevertheless his ‘public sin’ needed to both be seen to be disciplined, and that doing so would be of benefit to the man himself.