] But why couldn't Darcy just have referred Elizabeth to an un-named solicitor rather than a named co-guardian for verification? Elizabeth never checks with Colonel Fizwilliam as to the particulars and we do not hear from him again on the subject of Wickham.

It would be preferable to name Colonel Fitzwilliam as guardian because Darcy is aware that he is someone whom Elizabeth knows and likes and in whose veracity she would have complete confidence. It is also useful to the plot because at one point Colonel Fitzwilliam betrays uneasiness when Elizabeth asks him, "Does your charge give you much trouble?", and this later convinces her that at least part of Darcy's allegations against Wickham are true. Although Elizabeth does not check with Colonel Fitzwilliam, she could have:

"She was referred for the truth of every particular to Colonel Fitzwilliam himself -- from whom she had previously received the information of his near concern in all his cousin's affairs, and whose character she had no reason to question. At one time she had almost resolved on applying to him, but the idea was checked by the awkwardness of the application, and at length wholly banished by the conviction that Mr. Darcy would never have hazarded such a proposal if he had not been well assured of his cousin's corroboration."