First name to go with...

I like Carmen. It shouldn't matter if your Mum likes the names or not (though I understand it's easier if she does - she should never have said she doesn't like it in the first place, imo)

Was your grandma Italian? I really like Andrina but have only ever come across it in a series of books about a dancer that I used to love as a child (Drina dances series but her full name was Andrina due to an Italian father). I like it as a first name?

Other ideas:
Alexis Andrina Sarah
Bronte Andrina Sarah
Cassidy Andrina Sarah
Daphne Andrina Sarah
Eloise Andrina Sarah
Florence Andrina Sarah
Genevieve Andrina Sarah
Hester Andrina Sarah
Isabelle Andrina Sarah
Juliette Andrina Sarah
Katie Andrina Sarah
Leonie Andrina Sarah
Madeleine Andrina Sarah
Natalie Andrina Sarah
Olive Andrina Sarah
Phoebe Andrina Sarah
Rosalie Andrina Sarah
Scarlett Andrina Sarah
Tamsin Andrina Sarah
Verity Andrina Sarah
Willow Andrina Sarah
Xanthe Andrina Sarah
Ysanne Andrina Sarah
Zoe Andrina Sarah

Coralie Andrina Sarah
Violet Andrina Sarah
Ivy Andrina Sarah
Natalie Andrina Sarah
Miriam Andrina Sarah
Darcy Andrina Sarah
Louise Andrina Sarah
Meredith Andrina Sarah
Bethany Andrina Sarah
Eve Andrina Sarah
Beatrice Andrina Sarah
Harriet Andrina Sarah

Persephone (P-seff-eny) Andrina Sarah
Emily Andrina Sarah
Margaret Andrina Sarah
Charlotte Andrina Sarah
Rosalind Andrina Sarah

Ahh I read dm as dh. Carry on with Carmen then, I like it.
Andrina Sarah is very hard to find a first name for, I think the reason for choosing is great though. Hermione, Phoebe, Beatrice, Agnes (never thought I'd suggest that one), Roisin (row-sheen), Charlotte, Carys, Felicity, Laurel, Grace?

Elizabeth Andrina Sarah.

But with the greatest respect, Sweet pea. This is your and your DH's bsby. Not your mums. Shes had her day naming you and any siblings you may have so if you like Carmen. Go with it.
I personally don't like Orla. It looks like somethings gone wrong with Paula, I don't like Carmen either but its your baby.
I think Andrina Sarah are going to be hard to match to be honest as they don't exactly roll off the tongue and both end in a er sound but I understand they have to be included so I'll do my Best
Olivia Andrina Sarah
Sophia Andrina Sarah
Ellena Andrina Sarah
Mia Andrina Sarah
Lily Andrina Sarah
Vivienne Andrina Sarah
Katherine Andrina Sarah
Lucy Andrina Sarah
Elizabeth Andrina Sarah

Carmen - no
Orla as has been already said.
Marnie, Libby, Mary, Erin ..

Agree that there are too many similar sound endings in those names.
If dh hates Carmen, then no to Carmen!

Jane, Faith, Ivy, Heather, Lauren, Jennifer, Jade, Heidi, Estelle, Megan, Tamsin, Bronwen, Rachel, Clare, Kate, Annabel, Lucy, Lucille, Rhiannon, Amber, Elodie, Marin, Niamh.

If you do Orla, then each name ends with a soft 'a' (or 'uh' depending on where youre from)

Orl-uh andrin-uh sar-uh - too many a(or uh) endings