La Reserva is “deja vue”! Been there, done that…almost got the T-Shirt!
Re class action, the courts in Spain do not like them. As Suzanne says, every case is different, and the courts treat them individually. It does help if you use the same lawyer, as it cuts the ground work down for individual cases, and so for this reason, you may be able to negotiate terms. Obviously if there are people who have multiple apartments they would have to act separately.
I wish you all loads of luck.