We assess each character on its own. When I say "my dad is a real-life Romeo" (which is a comparison on the same level as "Jesus was a second Adam", hence the comparison) one cannot conclude from there alone whether or not my dad is historical or figurative or whether or not Romeo was historical or figurative. One looks at Shakespeare, finds that Romeo was a fictional character in one of his plays, and then concludes that Romeo is a figurative figure. Then one looks at my dad (who is very much real, thank God) and concludes that my dad is a historical (heh, heh) figure.
And then one goes back to the analogy and sees that a historical figure is being compared to a figurative figure.
Ditto the case of "Jesus is the second Adam". One looks at Adam on his own (and I have no problem with there having been a historical Adam), and then one looks at Jesus on His own, and then we go back to the metaphor and see what is being compared with what. The metaphor does not tell us any information about the nature of the people described, but that does not mean that information about them cannot be gained from elsewhere.