Every time I come across the subject of the Colonel, it frustrates me that I can never be sure what he really stands for in the novel, and I can't believe he is there for no reason!;

Funny what your post reminded me of. Years ago I asked the college professor who assigned my class the novel The Sun Also Rises, about one of the characters, Count Mippipopolous. He was, so the professor said, "used as a device to move the other characters around, as they followed him from party to party." It seems as though Col Fitzwilliam functions as a way to "move Lizzy around"---to fill in some gaps in what she knows about Darcy. So perhaps JA is simply using him as a device to impart information without her having to narrate it.