Isaac Gawain "Iggy" or "Zac" - I would think the parents are Merlin/King Arthur fans. Iggy is a bit rockstar, Zac is cute.
James Thorbjorn "Jimmie" - Would assume Thorbjorn was a family name. Prefer Jimmy spelling or Jamie as a nickname, Jimmie is a bit try hard to be different
Marcus Drake - I like this combination, my favourite of your boy names
Blaire Hyacinth - I don't like Blaire, I think Tony Blair who isn't a great namesake to have. Hyacinth I would assume parents liked the plant
Milena Cyrene "Lena" - I really like this too, cute and quirky without being too popular or out there.
Lilith Olwen Summer - I've neer heard Olwen before, not keen on any of the names though
Avery Idonea - I don't like AVERY, And how do yousay Idonea?
Isaac Gawain "Iggy" or "Zac" - I would think that the parents like classic first names with edgier middle names and are into cool nicknames, love Iggy!
James Thorbjorn "Jimmie" - Same as above about the first and middle name thing, I think the nn Jimmie for James is great.
Marcus Drake - Hmmmm... I like this one, it just doesn't seem as interesting as your other two.
Blaire Hyacinth - This one is okay, I like Hyacinth in the middle, but not sure if I like it with Blaire.
Milena Cyrene "Lena" - Pretty, this one is truly stunning. Milena Cyrene sounds very nice, love the nn Lena.
Lilith Olwen Summer - Lilith is pretty and even though I have never heard of Olwen I think that it works well together
Avery Idonea - Avery isn't my favorite by any means and I am unsure of how you would even pronounce Idonea.
For the boys names I get quite a rock music vibe. For the girls I think flowery and naturey.
Isaac Gawain "Iggy" or "Zac": I'm having a hard time with this one I love Gawain, but I still get a somewhat nerdy vibe overall.
James Thorbjorn "Jimmie": I picture a friendly, blond boy. Broad shouldered and good at sports.
Marcus Drake: I picture a dark complected, quiet guy. A bit mysterious.
Blaire Hyacinth: Blue is of course the color that comes to mind. Having a hard time coming up with a picture for Blaire just because my only experience with it was as a guy's name. It is also the surname of a good friend of mine. It sounds nice though!
Milena Cyrene "Lena": I picture a calm, blue lake. A quiet, but intelligent girl.
Lilith Olwen Summer: Lilith is still tied to the character from Frasier for me. This name gives me a dark, mysterious vibe. Summer would brighten it up a bit, but in the second middle position, it doesn't have much sway on the feel of the name.
Avery Idonea: again, the only Avery I know is a boy. Can't really come up with an image for this one. How do you pronounce Idonea?
All your name choices sound really nice. These were just the pictures that popped into my head first. My favorites are: James Thorbjorn (I love the nns Jim and Jimmie!) and Milena Cyrene. Isaac Gawain and Blaire Hyacinth are my second choices.