An "eternal optimist" means someone who is always optimistic. No matter how much time goes by--no matter what disappointments the person has--they're always optimistic. Angus Young (AC/DC guitarist) might be described as an "eternal schoolboy" the way he dances around joyously, dresses in a schoolboy uniform on-stage, and remains young at heart and full of fun and glee. "Eternal bride" is another example. The reader can decide what they understand by "the qualities of a bride" (shy, youthful, healthy, whatever), and therefore what qualities remain evergreen in Sophia. Doing a survey on whether people would or would not use a particular expression in real life might yield some information. But I think it's going to be more instructive to focus on exactly what the expression means and in what situations it would be appropriate to use it.

That's a tough one. The meaning is not obvious. But I think your guess is a good one: Sophia has the qualities (beauty, courage) that have long been ("eternally" been) associated with an excellent bride. But no, English speakers would never call someone an "eternal bride" in real life -- nobody would know what it meant.