] -- their families were doing some matchmaking to pair Georgiana and Col Fitz off together.

Plausible, but I don't think likely. He was significantly older than Georgianna, probably 12+ years.

Only Lady Catherine seemed pre-occupied with dynasty building. We hear nothing from Colonel Fitzwilliam's side on this. Only indication we get is that he marry with money, no mention of family?

] -- JA didn't multiply characters unnecessarily, and since she already had Col Fitz doing all sorts of useful things for the plot, gave him an additional task.

This makes sense.

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. I would have thought we might have had some explicit help from Colonel Fitzwilliam when Darcy was searching for Wickham and Lydia.

I suspect that Ms. Austen did not think overly hard on a secondary character to her novel. However, in case she did, maybe she was suggesting that these two co-guardians offered different talents and role models to Georgianna. Maybe Colonel Fitwilliam's openness and easy manners were a counterpoint to Darcy's internalization and concern for appearances?

For example, at the Netherfield ball, Darcy was disturbed by the unsolicited introduction from Mr. Collins but mabe the Colonel would have ignored the lack of societal convention and chatted pleasantly with the man about Lady Catherine. Georgianna could have learned from both of their reactions.

If Ms. Austen thought a lot about Georgianna, maybe she assigned two guardians because she wanted both perspectives; love and family devotion from her brother and little liveliness of manners from her cousin.

But, then again, maybe this was just a convenient plot device; keep the character list simple.