I really think that Gwen & Lilina should have the same class. I think that Lilina should be able to use the Durandal; after all, she is the one who allows us to get it. She's pretty much the only person with Roland's blood, so it makes sense canonically. IDK, that's just my thoughts. To make Gwen/ Lilina different, you could have Gwen be able to use light and Lilina use Anima.
I'd personally have Roy promote into Eliwood's final promotion, just so that he gets a sprite change. Plus, this allows for the use of Lances. You could easily have Gwen/Lilina promote to Erikia's promoted class, so that they have matching move with Eliwood.