Characters in other Austen novels get away without marrying their victims, even the ones they father children on. I think Wickham liked to manipulate foolish women, and the younger they were the easier to lead astray with his lies. I can see that more than I see him as being a man 'who had a thing for younger girls.' But the idea of Georgianna's story being foreshadowing is a good one, either way.

Something recently occurred to me, too: no doubt Mrs. Bennett gave Lydia spending money, and surely Wickham has no ready cash, and can not borrow any from his fellow officers who now know of his gambling. Is it beyond reason that Lydia's loose cash, meant for her months-long stay in Brighton, was being used to fund their flight?

"...she is luckily too poor to be an object of prey to anybody..."?