In the UK (and probably elsewhere), frozen fish is considerably cheaper and more common than fresh fish. In an ideal world, I would use fresh fish along with a cream or milk based bechamel sauce, spinach, cheese and potatoes then freeze any leftovers, but this is really expensive. How would a reheated frozen dish using fresh fish compare with one made using frozen fish? Assuming there are no intrinsic food safety issues associated with this practice, would the quality of the dish suffer from using frozen fish that was reheated twice? I've heard that most "Fresh" fish in UK supermarkets was already pre-frozen then defrosted, which would make it pointless purchasing it specifically for this dish.
I think your question needs clarifying, and better limited to a single question. However, freezing and thawing, repeatedly, particularly with fish, generally impacts texture in a negative way. There is not safety issue, per se. Given your preparation, you might not be too concerned about the texture, but that is probably the main issue.