I've just finished a Reformed Ragusa game in 1.7. So:
1. Decent/great stability because I was a merchant republic and had plenty of monarch points. I actually boosted stability several times paying 300+ ADM just because my monarch points were overflowing. Even so, 3 stability is incredibly rare, so 1 point at least is always "missing". You probably have 2 stability 50% of the time (maybe?). So another point is more like 0.5 => an average of 1.5 fervor points generated from the 3 you mentioned.
2. Most of the time I was at peace (building everything I could). But in an expansionist game you won't be at peace much => 0.5 fervor points.
3. I had 100 religious unity from 1600 on, since I only wanted to conquer a set number of provinces so no expansion after I had those. Also during conquests religious unity is not 100% (unless you have Humanism, which you should - and in this case things are a lot easier) => 1 fervor point.
4. My prestige was not necessarily 100 because of the relatively few wars, but once I built the unique building, I think I was around 100 most of the time after 1700. Still, keeping high prestige is not hard when you fight a lot => 1 fervor point.
5. Base value => 1 fervor point.
Extra notes:
6. Switching also costs a bit, as you said.
7. Religious grants 0.25 fervor points, and I think you should get Religious ideas.
1.5 + 0.5 + 1 + 1 +1 + 0.25 = 5.25.
Yes, later on you will have one focus all the time (or close to it, more like 90%), while the second focus is much rarer, I'd say 20% or less.
Yeah, that was just theoretical of course. But I found it to be very rare not to be able to run at least 1 focus. All in all I think reformed is fine. But Catholicism as the curia controller has to be nerfed. Those bonuses are ridiculous. Orthodox could be fleshed out some more, but the reworking of the PA events was a nice start.