“The answers to those ethical questions may change if you start treating some of the extra-canonical books as equally authoritative as the canonical books, so surely we do need to have a clearly-defined canon if we are to determine God’s will for our lives.”

Your approach here seems very pragmatic. It is also, I think, somewhat problematic. You seem to assume that having a clear-cut canon provides the Christian with guidance in life. Protestants have operated with the same settled canon for a fair few centuries now, and they differ wildly on what they feel God’s will for their life is. It would seem as though settling the canon settles nothing about the determination of God’s will for one’s life.
So then where are we left for the sticky issues you raise (abortion, homosexuality, divorce & remarriage, and pacifism)? To our own best judgment. No canon can ever take our responsibility away (I realize you aren’t claiming that it does), and so we must discern the heart of God. Like the ex-Muslims you describe, Christians must, through Scripture, discern the heart of God, and align themselves thereto. Can God’s heart be discerned from places outside of the Protestant canon? Probably.
I believe, as has been said elsewhere, that the Word became flesh, not that the Word became paper. Christians can therefore allow themselves to have a ‘bendy’ canon.

Peace,
 -Daniel-