Re Religion in RTW BI - it does have big impact. I don't know about RTW vanilla - maybe they added it in.

Having a ruler or governor with a different religious culture - e.g. Roman, Barbarian, Zoroastrian, produced religious unrest - sometimes severe. When a territory was conquered the original temple remained. The unrest intensified if you demolished it and built a new temple from your own culture straight away so there was a quandary about unrest and conversion rates.

It was a bit tricky to manage, and took longer, I recall, to reach a conversion level that lowered the unrest. However, the AI took into account the number of family members (generals) and agents you had in a province to modify unrest generally. There were no separate priest agents.

So, all those untalented family members ended up standing around behind the main army - while being careful their v&v's didn't upset things more. At least they had something to do ...