Cordelia Edith Jane and
Matilda Frances:
I like them together, although I'd want them to have the same number of middle names.
Cordelia Edith,
Edith Jane, or something.
Edith is the one name that really fits in with
Matilda Frances.
Matea is fine, although it's not exactly shorter. I think ma-TAY-uh is the only way to say it... I've never heard it any other way, and I can't think of any other way.
Sarah Mathilda Frances: I like it, but why add the h?
Mathilda is a bit more unwieldy than
Matilda.
Nathaniel Sherlock is kind of cool. I don't think it would work with a Sh--k last name though, even with another name in between. The other names you came up with definitely go with it though (except for
Ethan).