Does Hebrews 6:4-8 refute Calvinism?

Hebrews 6:4-8 is one of those "special" passages in Scripture from which a lot of debates between Calvinists and non-Calvinists have revolved around. This verse is frequently cited against the Calvinist (Reformed) doctrine of "Once Saved, Always Saved" since it mentions apostasy. Calvinists have long been bothered by this text and have sought ways to explain it, but I think the "interpretations" they come up with are pure desperation and ultimately undermine any responsible approach to the rest Scripture.

The passage states:

4 For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, 5 and have tasted the goodness of the word of God and the powers of the age to come, 6 and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. 7 For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. 8 But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.

The first thing I'd like to focus on is the start of verse 6 where this Protestant translation (correctly) says "and then have fallen away," since some translations (e.g. KJV) incorrectly include the word "if" such that it reads "if they fall away." This "if" is not in the Greek, and it's either added out of ignorance or out of an agenda to make this sentence conditional rather than an accomplished fact. (Some say the "if" was inserted by the Protestant Reformer Theodore Beza when he saw it refuted his Calvinist theology.) In short, the first thing to recognize in this text is that it is speaking of an apostasy that has taken place, not merely one that might or could. 

Recognizing this first point, since not all Calvinists I've encountered do, the Calvinist who knows better cannot approach this text a hypothetical, and thus they must explain it as someone who was never saved in the first place. But that begs the question and is a very dubious claim considering verses 4-5 cannot be describing anyone but a genuine Christian. And verses 7-8 support this claim as well, giving the analogy of a plot of land that after being watered (graced) can either yield good fruit (meriting heaven) or bad fruit (meriting hell). Lastly, any Calvinist pushing this view would seriously condemn their own assurance since they themselves could "experience" all those same gifts in 4-5 and yet it wouldn't be any indication they themselves were truly saved!

So they must then shift attention to the term "impossible," and from there argue that it cannot be speaking of actual Christians since repentance is never impossible. While it is true that one can always repent as long as God gives them the opportunity, the term "impossible" here can be understood different ways. For example, the term "impossible" could be hyperbolic, meaning used for exaggeration, reflecting how difficult or unlikely it is for someone who has abandoned Christianity to return (Cf "impossible" in Matthew 19:24-26). Or it could mean it is impossible to return to that once pure state you were originally baptized into, having to settle for an inferior/tarnished status among Christians (some great saints never committed a mortal sin). Or it could be referring to the "unforgivable sin," which I discussed on [this post]. Or it could refer to not being able to repent while in the midst of your apostasy, instead requiring some special pardon by the Church clergy (e.g. from excommunication). 

The following interpretation I think makes the most sense. First, the language of this text describing those apostates who "are crucifying once again the Son of God to their own harm and holding him up to contempt" strongly suggests this was an apostasy actually witnessed by the early Christians. Since this book was written principally to a Jewish Christian audience, or at least a congregation with Judaizing problems, then it's likely they witnessed apostate Christians fall into a Jewish lifestyle (cf Galatians 5:1-4). This 'problem' of 'crucifying Jesus once again' is held as the cause for not being able to repent, which also holds the key for properly interpreting the whole passage. As folks like Jimmy Akin explain, this text is saying these apostates were rejecting Jesus as the true Messiah and thus they were claiming He got what every false Messiah deserves as a fitting punishment and humiliation (i.e. Crucifixion). And once an apostate has gone this far, they're so hardened against Christianity that it's very unlikely ("impossible") they'll ever return.

To summarize the problems with the Calvinist approaches to this verse: (1) there is no IF statement; (2) the tone of the Epistle is practical and reflecting reality, not issuing empty threat "warnings" that are meant to scare but are basically misleading; (3) presuming that the gifts in verses 4-5 cannot be speaking of genuine Christians, when the opposite face-value reading makes the most sense. Ultimately with the last approach you can basically forget any meaningful exegesis from the rest of Scripture because at that point any passage could be "assumed" or argued to not really be speaking of salvation or true believers, which effectively puts traditions of men above the Word of God.