Personally I would go M W Shelley. Neutral as to sex and sounds more impressive, at least it does to me.

Then if she wants, she can always unpack other related names: Mary Shelley; Shelley Wollstonecraft etc.

Also there's a principle that literary fiction should use your proper name whereas genre can multiply pseudonyms as much as you like.

Remember you probably won't want to have a series of website names but just one where you can send everybody (and they might trip over other books they don't know about).

After using the pen name P F Chisholm for the Carey books and my own name for other books, I've now decided to bang them all together. 'Patricia Finney' and 'Patricia Finney writing as PF Chisholm'