Concerning Hebrews 6:4-8, the "preemptive question" is not invalid as long as it is not allowed to be "preemptive." In other words, if we are willing to suspend any effort to answer the "preemptive question" concerning Hebrews 6:4-8 and instead seek to understand the meaning and function of the passage, then the "preemptive question" will become a legitimate question in its proper place.
Thus, if we properly understand Hebrews 6:4-8 to be an authentic warning that appeals to individuals that the passage itself addresses as given the light of heaven, as having received heaven's gift of salvation, as having received the Holy Spirit, as having received God's Word, and as having become participants in the powers of the age that is coming, then we will understand that the passage serves to warn just such individuals lest they fall away to perdition. As a warning, the passage implies nothing concerning the "preemptive question." As a warning, the passage does not imply that it is possible for a genuine believer to fall away and perish, but it also does not imply that it is impossible for a genuine believer to fall away and perish. Warnings do not imply that the thing warned against is likely or unlikely to be realized. Warnings do not bear this function. Instead, warnings map out for us the assured consequences of following a particular course of action. Warnings caution that taking a particular course of action will undoubtedly result in assured consequences.
Warnings are not rendered genuine, serious, or significant by the likelihood of the thing warned against coming to pass. A suppositional warning or an admonition is established as authentic, serious, and significant by the truth that it projects. The truth a supposition projects entails two aspects--choices followed by consequences. Thus, if you follow this particular course of action, you will perish. If you follow this marked out pathway, you will not perish. Warnings and admonitions project assured consequences that will follow specified choices or actions. If you drink arsenic, you will die. If I, as a believer in Jesus Christ, abandon the way of the gospel to pursue a godless way of life, I will perish in my sins. This statement says nothing about the likelihood of my perishing. Instead, the statement does say that I will perish if I follow a particular course of actions or choices. The truthfulness of the statement does not depend upon the likelihood of it coming to pass for me. The truthfulness of the statement is established by the invariableness of the relationship between the action and the consequence that it projects as proclaimed in the gospel. The invariableness of the relationship between the action (failure to persevere in Christ) and the consequence (eternal perdition) is not mollified for me because I am a believer in Jesus Christ. This is precisely the point that Jonathan Edwards makes in the piece I cite from him here.
So, in the end, how would I answer the "preemptive question" concerning Hebrews 6:4-8? I would say that the answer to whether the passage implies that it is likely that a believer in Jesus Christ can fall away and perish is that the passage says nothing about likelihood of apostasy and perishing. Instead, the passage warns us not to apostatize lest we perish and find ourselves without any sacrifice remaining whereby we may be redeemed (cf. Hebrews 10:26).
I realize that my response may seem like a dodge or a duck. It is not, however, an evasion. At stake is the crucial acknowledgment of a rudimentary element of syntax that we are so inclined to misconstrue with reference to Scripture because we allow particular theological questions to prejudice our thinking grammatically. Suppositional warnings in Scripture, in themselves, function as suppositional warnings function in ordinary affairs of life. Suppositional warnings on containers of poisonous substances imply nothing whether you or I will take the course of action to ingest the poison so as to lead to the assured consequences of death. Such warnings aim at making sure that we never follow that course so as to perish. Suppositional warnings in Scripture imply nothing whether you or I will take the course of action so as to lead to the assured consequences. Rather, suppositional warnings in Scripture caution us not to take a particular course of action that will most assuredly lead to deadly consequences. Thus, warnings aim at preserving us from disaster. This is precisely what Hebrews 6:4-8 does, if we are willing to hear the text for what it says and not import to the text elements that we think we need to preserve the text from saying something that challenges our theological constructs and categories. Hebrews 6:4-8 does nothing to challenge, to displace, or to contradict what the Preacher affirms in 6:9-20, namely, that God will assuredly preserve everyone of Abraham's heirs without any loss, just as he has promised. God's warnings do not subvert God's promises. God's warnings serve God's promises.