What I am not sure about is why only priests can be founders,
If the religion was founded without priests, it would immediately start falling apart. (Yes, this does require the elder-priests things.)
enlarge temples
Not true. You only need to be an elder, IIRC.
and why half of elders must be priests.
This is to ensure that religions are serious about actually converting the peasantry.
If you can't convince enough people to be priests to keep the religion alive, then I'm sorry, but you don't really deserve to be a religion anymore.
