I keep thinking of Lydia Bennett hooking up with Wickham in Brighton as a model of what might have happened to Eliza.

We know that Lydia was inadequately supervised -- by her parents, who let her go, by her "hostess" who wasn't a chaperone but another young and giddy woman, and perhaps by Colonel Forster, the only grown-up in the house, who didn't notice that she was becoming especially attached to Wickham. In Eliza's case too it seems we have parental figures who are absent/negligent/over-indulgent, and a girlfriend with whom the girl can "range over the town".

More importantly, Austen doesn't depict Lydia as "prey" (though during the crisis, her mother describes her in sentimental terms as a victim of neglect and seduction). Rather she's a precociously sexual young woman... this registers in her physical type (a "stout, well-grown girl of 15"), her love of dancing, her man-craziness, her impulsive (even pushy) nature, her rowdy manners. None of this would INEVITABLY make her a "woman of easy virtue" or "fair game," but she's hot-blooded and responds eagerly to flirtation. She gets in deep fast because she wants to be there. I think Austen depicts young women with a wide range of sensual "temperatures", from the coldness of Mary to the apparent coolness of Jane and our Elinor (which masks their true warmth, of course) to the hotness of Lydia.

I picture Eliza at the time of her seduction as hot-blooded and unsupervised like Lydia. A dangerous combination. Obviously when Brandon finds her she is subdued -- as Lydia might have been if she'd gotten pregnant, and if Wickham had run out on her instead of taking her with him.