Kathryn. Catherine is good too, I just prefer Kathryn. Katie and Kat seem more natural as nn's for Kathryn plus I want to pronounce Catherine with three syllables and Kathryn seems like it should be two syllables, and I prefer the two syllable pronunciation. Also I love that Kathryn is the medieval spelling and it is the way that Captain Kathryn Janeway spelled her name in Star Trek Voyager. Plus I know the most awesome little girl ever and her name is Kathryn or as she calls herself "Johnny of the West". :)
Aveline. These are different names, Evelyn is pronounced differently than Aveline. I prefer the look and pronunciation of Aveline. Evelyn seems a bit old ladyish and dated but Aveline seems fresh, awesome, and it is unusual.
Luke. Luc is good as well, Luke just looks more complete to me.
Jack. Jacques is good too, I only slightly prefer Jack. I just think a one syllable name should not have such a long spelling.
Niamh. It's the traditional spelling and I prefer the look of it even if many people wouldn't know how to pronounce it.
Sean. Shawn is okay, it looks more immature than Sean. Shaun is my second favorite way to spell it after Sean.
Celine and Selene. I really can't decide between these two spellings.
Kristina. I'm not Christian but I've always thought Christ seemed like a cool guy, Christ as a name just is not very feminine and I feel that it is a little weird using a name that seems like Christ with ina stuck on the end. Also when I was a young kid I had a best friend Kristina and another good friend named Kristine. I was good friends with a Christina in middle school but we were only friends for a year before she changed schools, so I'm more used to the spelling Kristina.