I see Haven as a mix of nostalgic cottage core and something more edgy and artistic, maybe even grungy.
For middles, August is too wordy for me - like a place you for in August, and maybe too cool if that’s possible. I like Marion and Jean but the repeat N ending isn’t quite right for me, Jean is more complementary I’d say and gives it a southern charm - though Haven Marion is more 30s-40s style so that might be more of the direction you want and it does sound lovely. But I think Haven Luella and Haven Louise might be my faves, Haven Luella for more cottagecore and Louise, for more grungy.
Others:
Haven Eloise
Haven Beatrice
Haven Beatrix
Haven Lucille
Haven Lucy (don’t usually like Lucy but this is cute!)
Haven Juliet
Haven Delphine
Haven Lenora
Haven Ophelia
Haven Amara
Haven Celeste
Haven Tabitha
Haven Cosette
Haven Colette
Haven Marigold (better ending than Marion?)
Haven Penelope
Haven Clementine
I like Haven & Adelaine but I myself might go for something like Iris, Bryony, Beatrix, Ophelia, Vesper, Juno, Junia, Mara, Brea, Soleil, Romy, Tess, or Story as a first.