G'day exwitless:
Congrats on a thought-provoking topic!
FWIW I've always maintained that there's a heap of difference between those who are expelled because they were "found out" and those who left because they'd found the Borg was not "Da Troof". The way I see it, the person DFed is always liable to be taken back into the fold of the congregation and accepted as "a repentant sinner" (aren't they all?) whereas the person who DA has made that deliberate course which could only be changed by a complete reversal of conviction accompanied by a "I was so terribly wrong" confession. Even then, I doubt the JC would readily accept that and even if allowed back in, I'm sure their life would be hell as they live daily under watchful eyes and constatntly hear hushed whispers.
Of course, we can easily think of exceptions. For example, a person DF for say, adultery, may later be just as convinced about the real nature of the Borg as is the person who DA; in other words, we can't generalise.
I think it safe to say though that a person DA for the 'right reasons' will never be duped into going back.
The "unforgivable sin" ? Like you say, another in the arsenal of the Borg's scare tactics. You may find it interesting to know that the expression "unforgivable sin" is not found in the Bible (not even the NWT !) . What is there is this recorded teaching of Jesus:
"Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in this age to come" - Matt. 12:32 (NIV) In the context, this is referring to attributing the source of Jesus' miracles being Satan rather than the Holy Spirit. Much has been written about what Jesus may have meant but the WTS have taken this uncertainty by adding a certain 'mysticism' to it that has the effect of being a controlling tool.