There's also the metro and night bus. I usually stay at near Barceloneta metro station, 15-20 minutes by metro, about 30-35 if i leave before 5am and need to take the bus.
In theory, a person who purchased a ticket can launch a screen recording app, take a short video of the QR code, and then send it to a friend. The friend can then show this video at the entrance as in the Access Ticket app and thus gain entry. However, the ticket is also nominative, so in theory, if associated with a specific name, it could be possible to determine whether the person who used it first has already entered the festival or not.