If you need "Robin needs Lucina" content, why not have a chapter set after chapter 23 of the game, where despite not killing Chrom, Robin has just found out that he essentially is Grima. He could be terrified of what he might do, thinking he's a monster now, and feeling like all the pain Lucina went through, how much she suffered in her timeline, why she has constant nightmares is because of him. He could even be afraid that Lucina might hate him now, because he is Grima, the thing she hates above all else. Then you can have Lucina come in an comfort him, assuring him that he's not a monster, and that she doesn't hate him. Promises him that he's not Grima, and that he didn't hurt her, and that he's not responsible for what the other him did.