Firstly, in Personas 3 you can hide screen objects "properly" in a script - none of the messing about with transparent images needed. That's a big help.
As for the bigger problem, I would say the best approach, if it can be made to work, is to replicate the header data from screen 1 onto screen 2. If you can rearrange the screen elements so that the header fields have the same arrangement on both screens, it could disguise the fact that there are two screens. The detail table would just magically appear when all the header fields are completed.
The tricky part, as you say, is how to deal with the detail take changing structure when the header data changes. The only real way to do that, in this scenario, is to go back to screen 1, change the header data, and then head back to screen 2. That will lose any detail data that's been already entered. You can save that first and re-enter it when you get back. But if the table fields have changed you need to think carefully about how that happens.
Where the detail table doesn't change structure, this is fiddly but certainly possible. And I think you could do it in a way that looks and works seamlessly. That somewhat depends on the underlying transaction, of course. Where the detail table changes structure, I think your biggest problem will be in deciding how any entered data should behave when the structure changes.
Hope some of that helps 
Steve.