I don't think Wyn is a stretch for Guinevere at all. In the same style, I know a little Genevieve who goes by Vivi.
That is a cute nn! Genevieve is another of my favorites but one of my DDs is Niamh (pronounced NEEVE) so it's too close.
I love this name. DH does not, but I got to pick for #4, so this was her name for a few hours . . .until I heard my friend say it the way an English-speaker would. Then I just could not go through with it. I adore, adore, adore it in Spanish but it sounds so butchered in English.
Anyway . . .we are definitely on the same wave-length!
Penelope is a close second as my favorite girl's name, and it meets most of your qualifications, other than being a bit long. It is definitely easy to say and spell. (Because our surname is hyphenated and, thus, long, we are not using Penelope; we are using my other favorite, Matilda, which, unfortunately, begins with "M.")
Either we all have similar taste and are picky together, or DH is the picky one. I have suggested these to DH and keep getting vetoes.
FlyGirl, Linnea is a top choice for me. DH used to be OK with it (we were going to use it for #3) but now he says no. Sigh. No ideas from him, however!
Thank you, everyone!