I think you may be right (but maybe not the character).Since Harper's Island is loosely based on "10 Little Indians", then I have an educated guess on who it might be.
In the book, the real killer faked his own murder part way through the book.
I also think that Abby isn't Wakefield's child, but that the real killer is and was given up for adoption.
Therefore my guess as to the killer, is that it's.Booth