Great trip report! We visited Terra Mitica on a weekday in early June and the park was pretty much as you described which is virtually empty. As a matter of fact, we had to wait a good amount of time for Inferno - not because of the queue but because we needed two other people to show up!! Although we enjoyed the low crowds, it is hard to imagine if this place will make it. The theming here is absolutely incredible, and Terra Mitica was our favorite park in Spain (we visited Port Aventura, Parque de Attraciones Madrid, Isla Magica, Tivoli World and Tibidabo).
Was the SLC still down for your visit to the park? It was completely roped off when we were there. Also, one of the major water/dark rides was closed but the name escapes me.... we were pretty disappointed about that. The Labryinth of the Minotaur is an incredible dark ride and the best example of an interactive darkride out there. We just couldn't get enough of that. When I first heard it was "interactive", I was disappointed but blown away after riding it.
By the way, it was hot as hell when we were there - it must always be like that in Benidorm during the summer? The park doesn't have much shade which is the only thing - there were a few times when I thought I might just burst into flames!