There are two things I would like to point out.
1 - P'varism is not the only path to wisdom. It is simply the most influential and institutionalized. It does not have a monopoly on the truth, nor on the minds of the arcane community.
2 - To use a crude analogy, P'varism lists the ingredients, but it does not provide all the recipes. It provides a framework of elements - a vocabulary for describing arcane phenomena - but does not describe all the possible processes by which they can be observed and manipulated.
So very true. In my pHarn, P'varism is just one way of describing magic. My elves see magic somewhat differently, and so do not bother with the convocations, but this makes it harder to learn. I also have runemasters in Ivinia, and Conan-style evil sorcerers are more common than P'varic mages from Dalkesh eastwards. As I see it, P'varism not only has restrictions against summoning demons, animating the dead, possession of souls, etc., but it has no method of doing so. These types of magic are outside the scope of P'varism, but they still exist.
I also separate religious magic into two types. First, there's the stuff like blessings, baptism, weddings and funerals. Any priest can do these, and all are expected to know how to do them, but they have no material affect on the game world. They are ceremony, not spell rituals. The people feel better when blessed, they think they now have a benefit, but it's all mental. As you may guess, I don't use the Piety attribute.
Being a priest is a job description - they can learn carpentry, cooking, and magic on the side, but get none of these just because they are a priest. A priest that can cast a real spell is quite rare, and some religions actively discourage it. Some may be P'varic mages that have entered the priesthood. Some are priests that have attended a chantry in their spare time. Most are priests that have been taught one of a very small number of spells that are common to that church. They consider them rituals, and call to their god during the casting. The spell always has something to do with the deity's powers (Agrik - fire, Peoni - crops, etc). However, these are no different than any other spell - they are simply channeling power from the ether. If an Agrikian priest learns how to summon a wall of fire, it's likely to be quite similar to the wall of fire a Peleahn mage creates. They describe it differently, they learn it differently, and they may even think it is different, but it's essentially the same thing. The temple may only teach 5 real spells, but they teach very little of the basics and theory of magic, and do not teach how to research and learn/create new magic. They have a limited cookbook, and they know if you can follow the directions correctly, a certain result will happen. Some know it's just another magic spell they are learning, while others go to their grave thinking they were channeling the power of the deity. Most priests never bother to learn, or never get the chance. Learning the small number of available spells/rituals may or may not help advancement in the church hierarchy.