I guess that overall, we are not so catholic here, Easter week seems to be more religious for the south of spain (Andalucía). I think these days people will take it as normal holidays. So, maybe some of them will make some trips (and PortAventura may be one of their targets).
For holidays - summer vacation / high attendance, sometimes is better to go weekends, but only for summer. Usually, the wednesday is one of the weakest days for the week. Maybe some other people here could help you at this point better than I.
We don't have yet any info of Templo del Fuego ride, but maybe it could be open only during peak season, I think we should wait for a while to know.
About the park attendance, I can't tell too much, just I expect the park have more guests, but not to be full. If you want to ride Shambhala, I would recomend you to wait at least for mid june to be sure the ride is open (expected opening for may/june)
You can contact the park calling 00 34 977 90 90. (german number, maybe they spek german here) or 00 34 902 20 22 20 (spanish number). You can attempt to contact filling this form http://www.portaventura.de/kontakte
, but I would recommend you calling by phone instead, as they can take too many days to answer a mail.