I really like your suggestion of Lucy & Seren. I don't like Bryony, Lavinia or Merryn at all. The others are lovely but, with the exception of Tabitha (possibly a bit witchy?), too popular for my own tastes.
Lucy is a very pretty, girly name so I think you ought to pick something that compliments it. I really like Cora/Dora/Nora but I can't help but think they are a bit 'granny chic' in comparison to Lucy.
Using your list as a starting point -
Instead of Amelia/Emily/Grace: Lucy & Alice, Ada, Aimée, Alice Amélie, Angelina, Audrey, Carys/Charis, Cecily, Celia, Claire, Clara, Clemency, Elodie, Eliza, Eloise, Eden, Esmé, Esther, Emma, Evelyn, Faith, Felicity, Fiona, Helena, Honor, Hope, Juliet, Ottilie, Sophie, Verity, Violet, Vivien, Willa, Zoe
Instead of Bryony: Lucy & Bethany
Instead of Lavinia: Lucy & Eleanora, Lydia
Instead of Merryn: Lucy & Corinne, Erin, Karen, Maren, Meredith, Miranda, Miriam
Instead of Cora: Lucy & Caroline, Coralie, Dora, Flora, Isadora, Nora, Pearl