The thing is Andrew, that it's not just that the Bible is metaphorical in places, it's plain wrong in places, Paul's assessment in his second Adam argument, isn't metaphorical. Paul is proposing a sort of mechanism which just wouldn't work as a metaphor. Whether of not it's 'core' will depend upon your perspective, but it certainly means that at least some of Paul's theorising isn't correct - one can fully understand that Paul would build his arguments upon the things he knew, but it is the work of a man, and not the work of God.