G
G asked Julia Quinn:

This question contains spoilers… (view spoiler)

Julia Quinn I figured it out at the end of writing The Duke and I. So I basically wrote that whole book without knowing who it was. When I DID figure it out, I went back through the manuscript with a fine tooth comb to make sure there was no reason that the the person I'd chosen couldn't have done it. (IOW, I had to make sure the person was always in the right place at the right time.)
Julia Quinn
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