How much worse punishment, do you think, will be deserved by the one who has trampled underfoot the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace?

It does. But honestly, it being Paul ;-), I think he's being tongue-in-cheek, saying, setting aside just the Law results in death; how much more terrible punishment for those who set aside the blood? Otherwise you have to deal with the first part of that statement--the blood of the covenant by which he was sanctified, i.e. the person was de facto sanctified by Christ's blood. If that really means that, and not just to create a juxtaposition between what the person assumed (Christ's blood) and abandoned; then Christ died for that person and it was not limited atonement after all. I'm not sure how you can read the passage that way. I think Christ's blood was fully efficacious for all the elect and only the elect, and so Paul has to be making a figure of speech here, rather than contradict the whole of the rest of the NT. The "blood" part is a problem before the "sanctify" part is.

This is essentially a side issue, though, because as /u/bsmason points out, there are many, many passages that we're looking at through different lenses and I don't think either of us is going to be convinced solely on the basis of Hebrews 10.

My real question is, it sounds like you're saying people are in the New Covenant and inheritors of its promises unless they fall away, which sounds like essentially reverse Arminianism. And I'm not trying to be snarky, I'm really confused. If people will to fall away, if perseverance is synergistic, I really, really don't understand how that isn't salvation by works.