I think Leni sounds cute with Lotte. I think I'd want it as a nickname though - Adeline? Adelina? Helene? Helena? Lenora? Leonie?
I love Grace and don't think it's overly common for a first name.
Mathilde. I'm not sure about. I really love Mathilda but just not sure about it without the a ending for some reason.
Adelie - never heard it before but it's really sweet, I like it
Hilde - is this pronounced the same as Hilda? Or is it Hild? Or Hild-ee? Don't like any of them really, tbh.
How about Lotte and:
Astrid, Ada, April, Avril, Anais, Alexis, Ariana, Adriana, Allegra, Aria, Alayna, Alannah, Bronte, Belle, Bianca, Claire, Clarice, Celest, Cecile, Cecilia, Cecily, Claudia, Claudine, Cassandra, Coco, Cleo, Delphine, Daphne, Desiree, Demi, Elodie, Eloise, Erica, Elianne, Elina, Elise, Felicia, Florence, Giselle, Gabrielle, Genevieve, Honor, Heidi, Ines, Iris, Isolde, Juliet, Josephine, Livia, Laurel, Leonie, Larissa, Madeleine, Marianne, Melinda, Marissa, Melisande, Maria, Margot, Miriam, Mara, Nina, Nadine, Nathalie, Ottilie, Odette, Odille, Peronelle, Romilly, Rowena, Rhiann, Sylvie, Sienna, Sadie, Susannah, Saffron, Sorrel, Tamsin, Tamara, Thea, Vivienne, Vanessa, Verity, Yasmin, Yvette