I find your use of the word "Evangelicals" to be curious. I hope that you understand that "Evangelical" is a standard term used for all Protestant churches, including the Evangelical Lutheran Church. Do you include yourself in this term?

Yes I'm aware of the historical usage. I'm referring to American Evangelicals, plain vanilla Protestants found somewhere along the Baptist-NonDenom-Charismatic spectrum. In this discussion, I'd even include Independent Fundamentalists.
Ultimately, of course, it is God who determines one's salvation. What assurance one has comes from scripture. There are those (not myself) who believe that once a person makes a profession of salvation then he has a complete assurance of salvation regardless of any future sins on his part, including, as you mentioned, complete and willful rejection of the faith.
This sword cuts both ways. Many Lutherans, having been baptized as infants, rest calmly in that assurance of their salvation even though they may grow into determined sinners. Likewise, many Baptists (I will pick on them because they are credo-baptists and not paedo-baptists) rest calmly in the assurance that they are saved because they recited a prayer. Neither of these possesses actual faith in Jesus Christ, but, rather, a distorted faith either in baptism or in saying some words.
I agree that it is God's job to divide the sheep from the goats, to decide which of all us sinners he is saving. However, my tent is probably a bit larger than yours since I believe that it is God who does the saving, and we are all equally wretched sinners, beggars before God.
In my mind, probably the best question to ask a potential apostate is 'do you still believe in Christ for your salvation?'
You did not address the portion of my question that deals with someone who is a devout believer for a significant period of time, then ultimately rejects their faith and embraces anther religion or atheism, however.
I think the reality is that the promise of Jesus Christ to never allow anyone or anything snatch His elect from His hands is true. To think otherwise would either make Him a liar or, at best, horridly deceived.
Before God saves us by gifting us with saving faith, our will is bound to our sinful flesh, but afterward we do have the ability to reject the gift once given and received. I find it scripturally inescapable that 1. God alone saves, and he will not let anyone take away his gift or wander away without being pursued and rescued, but also that 2. God allows us to reject his gift of saving faith if we decide to embrace unbelief.
See the parable of the sower - those believe for a while, but ultimately are lost.
In other words, scripture teaches that grace is irresistible, and that once saved, we can resist grace. Yes, this isn't logical, but it is scriptural.