Hang on. I just thought of a way to have the younger Luke and Leia flashbacks without technically going against what they said about not casting a new actor in the role: have Billie Lourd do it. If the flashback is what has been reported - the twins post-Return of the Jedi sparring in some kind of helmeted armor - it would be easy to have Lourd play Leia's part and use both makeup and digital manipulation to enhance the resemblance to her mother in her mid-twenties.

Lourd is even the same height as Fisher. o_O

I'm not really into the idea (especially since "Leia reveals to Luke that she's pregnant" is yet another way of centering Kylo in the narrative), but it's possible to do without bringing in someone else who hasn't been in the sequel trilogy.