Miriam is very nice. I used to not care for it, but it actually has a sweet name, and I do adore Miri as a nickname, but I agree that it wouldn't work with
Mira around. However, if you used Miri as just an occasional nickname in your household only, it may just work.
I like the idea of honoring, so I suggest
Miriam Elizabeth, but I'm not sure how you would feel about that.
Miriam Ivy sounds wonderful to me right now.
I don't like the sound of
Mercy, and
Miriam and
Mercy don't work for me.
Miriam &
Helen on the other hand sound lovely to me : )