Congrats on the pregnancy! <img src="/images/graemlins/smile.gif" alt="" /> Here are some thoughts on your list of names:
Christian - I'm personally not a fan of this name. (To be fair, though, I'm an atheist and have uncommonly strong opinions about religious names...)
Mason - Like it. I think Mason Edward would sound good.
Cameron - I used to like it more, but it's being used more and more for girls now, and I don't really like when names start crossing over from one gender to the other.
Blaise - Hm... I'm not a big fan of this one.
Killian - I like this a lot. Though a boy named Killian might get teased because of the beer association (Killian's Irish Red, which, incidentally, is my mom's favorite. <img src="/images/graemlins/smile.gif" alt="" />) But, other than that, it's a really strong, masculine name. I think it'd sound great with William.
Keiran - Another great one.
Jackson - And another.
Jayce - I'm lukewarm on this one. It's not bad, but I like other names you've listed a lot more.
Connor - Like it. Maybe Connor William.
Dawson - Like it. I think Dawson Edward would sound good.
Rory - This one I've heard on bother genders (like Cameron), so I don't care for it.
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Ava Ryann - I agree that Ryann sounds better that Eileen would for Ava. I think it's because the vowels sort of slur into one another (Avaeileen) when it's said aloud. Ava Ryann is very pretty.
Caoilainn Eileen - I think people will have a very hard time pronouncing Caoilainn. It might be a frustrating name to have (unless you're in Ireland <img src="/images/graemlins/smile.gif" alt="" />). It might make a better middle name for this reason, imo.
Miranda Eileen - Very mature-sounding, to me. I like it. Down-to-earth, not frilly at all. I think some people might think it a little stuffy, but I think it's a good combo.
Shalyn Eileen - I've never seen Shalyn before. It's cute, but I don't like it as much as some of the other names on your list.
Cortney Eileen - Pretty, though I prefer the traditional spelling, Courtney.
Joanna Eileen - Not bad. Thought Joanna has a dated sound to me. I think it was a lot more popular a few decades ago -- whenever I hear the name Joanna, I think of legwarmers. <img src="/images/graemlins/smile.gif" alt="" />
Trista Eileen - Trista is really cool. I like this pairing a lot.
Koralyn Eileen - I don't care for the way the names echo each other ('-alyn' & 'eileen').