I do think Elliot and Elinor are to close for siblings. I am not a fan of Archibald but I do like Archer. Elliot and Archer work well together. From you lists of names, I really like Oscar, Adam, and Gabriel. As for your girls names, Elinor is my favorite but I do think it precludes using Elliot. With Elliot, I like Adele, Maxine, Rose, Hnnah, and Cleo.
I like Elliott, not sure on Archibald but they do go together well. What about Archer? I love the nickname Archie. Very sweet. I would give them fresh, modern middle names to prevent the name from being too heavy.