What is meant by the impermissibility of an individual praying behind a congregation: does it mean that one should not pray alone in a row behind a full row, or that one should not pray individually while the congregation is praying?

The prohibition against praying alone behind the row refers to an individual praying by himself when there is a row of worshippers present, regardless of whether there is an empty space in the row in front of him or not. Whoever comes upon a congregation performing a fard (obligatory) prayer must join them if he has not yet performed that prayer, or if he performed it alone and it is one of the prayers that can be repeated. Congregational prayer is obligatory, according to the preponderant view among scholars. And Allah knows best.