How would it be possible for one of God's ELECT to reject the doctrine of unconditional election?

Because even the elect are not delivered from the sin nature until death or the rapture, we still struggle to submit our wills and reason to revelation. Unconditional election is a man-made term - you will not find those words in Scripture and even though it is a Biblical doctrine, many wish to reject it in favor of retaining some of their self-determination. The Hebrews page that you cited, I do not believe, means that the entire Word of God is in a believer's mind - that is why we must memorize it. But when one encounters the truth in the Word, there should be a response that affirms that it is true - it is "written" in the mind. One can be genuinely saved and still not understand the entirety of Christian doctrine, especially the tenets of the Reformed faith.