Yes, I agree. I hadn't looked at the context of Heb.3:14. Seems we are on the same page. However, when one thinks about it, if a person abandons faith and departs from God, then for that person the subject of a guarantee becomes irrelevant. At that point, they are not interested in a guarantee.
An underlying theme in Hebrews is faith. Even in ch.6, we see that the foundation for a Jew was to repent from dead works and turn to faith. The works of the law were and always will be dead works when they are conntected with justification for life. So, the danger for them and for many now is to turn from faith and head towards self justification through works of moral obedience. Heb.10:39 assures them that they were not of those who draw back to perdition but of those who believe to the saving of the soul. If this isn't taken into consideration along with the beginning of the context, then vs.26 will be misunderstood. Even in that chapter, the advice is repeated (vs.35), to not cast away their confidence. The author had just given a lengthy and detailed teaching concerning the accomplishment of the sacrifice of Jesus and its superiority over the blood of bulls and goats. Obviously they needed to hear all about it and get a clear understanding. The warning for them was to not go back to the old way and old covenant. Under that system, there no longer remained any sacrifices. As you have so rightly pointed out. Context is imperative in understanding a text.
People will isolate vs.26 and use it to warn others to behave themselves or else lose their salvation. Like I shared. They exhort others to do the right things but for the wrong reason. If a person is depending on his own behavior to get or remain saved, then he is outside of the confidence that the bible and God has granted to man. You are correct, We have the free will to do that. However I wouldnt be quick to write off a person who has been deceived. There is this factor of " forgive them for they know not what they do".