Nimue and Vivienne and Ninniane, Niniane, Lenore

Ninian - an esteemed male aka Sir Ninian Stephens
Vivienne is lovely
Leonore is my fave of your list.
Nina

I think of The Lady of Shalotte

@NoisyDachshunddd I very much think that 2% will be the ones benefiting from you swerving them.

How judgemental, there are some names that people have never come across before or ones with several different pronunciations

My 73 year old mum is Lenore so I'm biased towards that, although love Vivienne as well.

My mum pronounces it Len-or, her nn is Leni - despite growing up in the 80s/90s there were very few references to the fabric softener, except for one kid who said "Lenore? I bet she lives life in comfort."

Lenora

I would go for a combo of your two - Lee:OH:norr

@Zampa , I hear it in an english accent accent as Leo-nor (non-rhotic) and in a a spanish accent as Leh-OH-norr (norr like in Norris)

But they sound they same pronounced in English and Spanish 🤔 (to me, at least).

Why would anyone say them in Spanish?
Leonor is nicer in Spanish.

Well, i think they sound better in Spanish.
One of my daughters best friends is a Nimué (it is pronounced Knee-moo-éh) with the strees on the last syllable. I quite like it. She is the only person I know with that name.
Leonor is the name of the Crown Princess, it is medieval sounding here, like a bit retro.
Vivienne I find would suit a woman born in the 50s.
Your other choices I find a bit meh and out there. I would use the first one.

We went with a Welsh name which we constantly have to spell and pronounce for people
My experience, with an Irish name working in the UK is that it takes a lot of people a lot more than one hearing to get an unusual/foreign name. I have had lots of experience of my name being mangled, and for those who are happy to ignore people who can't pronounce their unusual names, this isn't possible with colleagues and clients.
This.
Even with names you think won't cause much problems.

I LOVE both Nimue and Vivienne!! Really beautiful

My experience, with an Irish name working in the UK is that it takes a lot of people a lot more than one hearing to get an unusual/foreign name. I have had lots of experience of my name being mangled, and for those who are happy to ignore people who can't pronounce their unusual names, this isn't possible with colleagues and clients.

Vivienne and maybe back to the King Arthur drawing board for another name.

I really liked Nimue after watching Cursed while pregnant with DD, but as much as I like it, I’m glad I didn’t go with it. We went with a Welsh name which we constantly have to spell and pronounce for people, a name like Nimue would have been too much.

I love Vivienne though, that was also on my list.

I like Nimue ( Nimmi) also Viviane. A friends daughter Vivienne is known as Vivvi rather than Viv, if you are thinking about nicknames maybe that might work?

Inghean in Early Modern Irish would be "inyan", it was shortened to "iníon" in 1958.
Ninian was a man's name used for a long time.

There's a River Ninian in Brittany.
How is Inghean pronounced?

Wonders idly if Ninniane is "an inghean" (Early Irish meaning "daughter", apparently from French "ingenue" meaning young woman with semantic overtones of innocence, naivety etc.)
But I am not an Arthurian scholar. Sure this is Mumsnet so one will be along shortly to enlighten us.

If anyone tells you that your baby’s likely name is “AWFUL” you should probably ignore them as that’s a pretty horrible thing to say to someone

-My baby is called Balonz
-What an awful name!
Horrible

-I'm pregnant and going to call the baby Balonz
-What an awful name!
Horrible

-I'm dithering between Tom, Dick and Balonz for my baby what do you think?
-Tom's lovely, Dick is OK, Balonz is awful
Not horrible

Nimue is a skincare brand. I like Vivienne the most.

Nimue? Awful.

Vivienne would be shortened to Viv which is not good.

I'd avoid it on that ground. I think if it was a Soph-ee-a/Soff-ya/Soph-eye-a type name, then I probably would say use it, but maybe not if some of the ways people might say it are not pleasant sounding.
With Nimue then I'd say avoid.

Nimwen just looks wrong.

Here's the catch - she might care or she might not care. We don't know and you don't know.

@oldnorsesaga , it's not a non-issue. You might not care but Nimue might mind. Imagine being Nim-oo-ey, Neemu, Nimway, Nim-ewe, Neem-oo-y, Nimmoo etc and it would be a PITA.

I have a completely phonetic name that's easy to say (about as easy as Tom), and I mind when people get it wrong.

The spelling and the name Nimue only exist in one late version of the Arthur legends. In older versions you get Viviane, Ninniane etc. And those names seem like scribal copy errors - someone mixed up V and N, or U, or M.
The name Imogen is a copy error for Innogen, for comparison. With Imogen, the similarity to Imagine and the straightforward phonetics make it a popular name.
Nimue isn't standard spelling in any language ttbomk. It does sound warm that's why I suggest Nimwen which is an easy spelling to read.