I think Dahlia is really pretty! Another idea is Delilah, they sound alike, but personally, I like Dahlia more, just wanted to throw the other out there.
I'm not a fan of Darling, it's cute for a little girl, but how about an adult? Maybe not. Also, if you name your daughter something darling, what if you have another daughter? Would you feel the need to include another cutesy name?
Dahlia is perfect! Dahlia Pearl and Dahlia Josephine Marie/Pearl are beautiful. And Dahlia is such a neat and stylish way to honour a Darl.
I also forgot to say, Louisa/Louise are lovely with your names too, if you think they were Darl's middle names. Using Louisa/Louise and Marie together would be nice as you have both your grandma's middle names in there- maybe Josephine Louisa Marie or Caroline Marie Louise or just Marie Louisa?
Woo - sentimental teenagers!
Honestly I think Rita is lovely. It has such a lovely old-Hollywood feel. What about Margarita? Then you sort of include Rita and Marie? If you wanted to honour her with Marie without using Marie, what about another one of Mary's variants? Maire, Mairi, Marion, Miriam, Mara.
I've always had a soft spot for Darla. I just think it's lovely (I also quite like Darling, but that's pretty clearly unusuable). What about Dahlia?
Zosia - ZO-sha, is the Polish form of Sophia (I feel like I'm always pushing Zosia onto people, I just want it to be used more), which *sort* of honours grandmother's Polish heritage and your other grandmother's middle name too.