At the end of this process, I realized that the database backup had been truncated near the end. The backup contains the entire table 'xf_user' but cuts off near the beginning of the table 'xf_user_alert'. The backup mostly restores correctly, and I can verify the presence of all the appropriate tables and rows (before xf_user_alert) via phpMyAdmin. However, with this backup I cannot log in as an admin, which seems as if it would necessarily be the first step of any recovery.
In retrospect I should have run the install to a new database and let the old one remain. Failing this, however, is there a way of recovering from the partial backup? It seems pretty clear that all the relevant data is present, of posts, threads and users anyway.