"The casting level of the resulting spell effect is based on the primary caster. " is the sentence to which I assume you are referring.
Out of the three alternatives, neither skill rank based casting level nor PP based casting level makes sense for rituals (at least as written), so yes, the default of casting level equals primary caster level seems like the way to go here.
This leaves us strangely limited in trying to, for instance, dispel a powerful enchantment with rituals. In theory, I would consider something like this a prime candidate for reasons to perform a ritual instead of regular casting. You are able to boost all the other parameters of the spell after all. On the other hand, it would go hand-in-hand with the fact that a default target of 50 for resistance would have a lower expectation value than normal casting.