The best on your list, for significance sake (i.e., [name]Willa[/name] Cather), is [name]Willa[/name] with [name]Harper[/name]. It is feminine, but not too feminine, like [name]Viola[/name] is (with [name]Harper[/name], that is). [name]Peyton[/name] would be a natural choice, but it seems like maybe it isn’t the best…
You need a semi-unisex name that has substance, in my opinion:
You could use [name]Halsey[/name], after [name]William[/name] [name]Halsey[/name], a U.S. Naval Officer who was heroic in WWII. [name]Halsey[/name] and [name]Harper[/name] would be cute together, as their sounds are different enough, if you are comfortable with them having the same first initial.
I’m not sure if you are a Wuthering Heights fan, but the novel was written by [name]Emily[/name] [name]Bronte[/name]. However, she published the novel under the androgynous pen-name of [name]Ellis[/name] [name]Bell[/name] (Her siblings used the pen-names of Currer - [name]Charlotte[/name] - [name]Bell[/name] and [name]Acton[/name] - [name]Anne[/name] - [name]Bell[/name]). [name]Ellis[/name] would be a great name with [name]Harper[/name], too.
You could use [name]Emerson[/name], for [name]Ralph[/name] [name]Waldo[/name] [name]Emerson[/name], and that works with [name]Harper[/name].
Finally, my last suggestion is [name]Bellamy[/name], a relatively undiscovered surname ([name]Craig[/name] [name]Bellamy[/name], a Welsh footballer, Manchester City club). [name]Bellamy[/name] has the advantage of having two cute nickname, [name]Bella[/name] (or [name]Belle[/name], I suppose) and [name]Amy[/name], and it works with [name]Harper[/name].
Unfortunately, none of my suggestions really work with [name]Eleni[/name] as a middle name, and it isn’t even the best with [name]Willa[/name] because of the competing vowel sounds ([name]Willa[/name] [name]Eleni[/name] requires a big pause between the two names…). For [name]Halsey[/name] or [name]Bellamy[/name], you could switch the name to [name]Elena[/name], as [name]Halsey[/name] [name]Elena[/name] and [name]Bellamy[/name] [name]Elena[/name] work well (especially [name]Halsey[/name] [name]Elena[/name]). For [name]Ellis[/name] and [name]Emerson[/name], you may need to start anew in the middle name game…
Good luck!