Although it's hard not to talk about very special future events and your plans for them, it might be best in this case to stop talking about your name choices to anyone you think will be critical. If this family member brings up the subject of baby names when you haven't, try saying things like "Well, April is still a long way off!" and change the subject.
As for the names you're considering, I wouldn't use Lilith or Cillian, but the others are perfectly good names with positive connotations. (I especially like Melinda and Rosalind.) I can't imagine that anyone with one of those names would hate it or want to change it. This relative is evidently just projecting her own prejudices. As for her suggestions, do you think she's serious?!
If, when the baby arrives, she really does try to call him or her by some name of her own choosing instead of the name you and your fiance have chosen, you might tell her politely that you can't risk confusing the baby that way and can't bring it to see her if she's going to do so.
BTW, how does she feel about her own name? If she doesn't like it, then the next time she brings up the subject of the baby's name, try saying, "Well, if it's a girl, maybe we'll just name her for you."
Finally, there's something you can do between now and April to give your baby a more stable family situation and a definite LAST name. You know what I mean! And then this relative can go postal over things like your choice of colors, music, reception food, etc.