Gwen and Guin sound different in many accents, so I don't know if Gwen could work as a nn for Guinevere, and Gwenevere looks wrong.I love Gwendolen, so I would use that instead.
I had the same thought about Gwen and Guin, Guinevere and Gwenevere. That's were my hesitation comes in, but I love the name. I do also love Gwendolen, just slightly less than Guinevere.