Congratulations Blade! Your list is great.
My favorites from your list are (and all of them would go well with Antoine, though some better than others):
Ambrose- I like this name a lot (though I prefer Emrys). You would be having the two 'A' name kids, but if that's not a big deal for you, then this would be a solid choice. Doesn't go quite as well with Antoine as some others, but would work. I personally prefer it as a MN though.
Cassian- I think this would be a great choice, and works really well with Antoine.
Cyprian- Same for this one.
Edmund- Doesn't have quite the same flair as Antoine, but it would still be a great choice.
Elias- Pairs up with Antoine a bit better than Edmund, but not as well as Cassian or Cyprian.
Evander- As you well know, I really really like this name, so I'm pretty biased about it. It's one of those names that I feel could fit with most sibsets. I feel it goes great with Antoine, though Leander would perhaps work even better.
Julian- A great name, though perhaps it doesn't work quite as well with Antoine as some of the others.
Leander- This one works exceptionally well with Antoine. There is something about Antoine and Leander that makes me think they are brothers. Leander is a little softer than Evander, which might make it match up with Antoine even better. It's a great name.
Lionel- This one would also work very very well. Both Antoine and Lionel have some sort of (I don't know if I can quite explain it) like 'holding back' on a vowel sound, that makes them work really well together.
Lucian- Another great name. Would work very well.
Malcolm- I like this name a lot, though perhaps it doesn't match up with Antoine quite as well as some of the others.
My stand-alone favorites (though all of them are great), not considering how well they match up with Antoine, would be:
Evander, Julian, Lionel and Lucian
But thinking about them paired up with Antoine, my favorites for you would be:
Cassian, Cyprian, Leander and Lionel
And my favorite of your middles would be:
Gratian (could be a good FN too)
Rainier (could be a good FN too)
Sylvan (could be a good FN too)
Some other possibilities (I don't know if these would meet all your criteria or not):
Altair (as FN or MN)