Didn't read the whole thread; sorry if it was already said.
Tip: you can usually have any, and possibly even all, religious buildings in at least some of your cities - WITHOUT having corresponding followers' belief.
How to:
1. manage your city to have majority religion being one of AI's religion which has a follower belief (or two) which allows to build religious buildings (you can check that in religion overview);
2. buy 'em buildings right away;
3. convert your city back to your own religion;
4. the buildings will stay. Profit!
To do p.1, either
- capture an AI's city which has majority religion being the one you need, then buy a missionary there (or few of 'em for future use). Note, ONLY missionaries will do; great prophets will always be of your own religion, no matter in what city you buy them. Then spread religion with those missionaries to your cities. Remember to move any inquisitors away for 3+ tiles, if you had any, otherwise you won't be able to spread. This works best for new cities which you just create. Note, even if you're peaceful type, - a small war and 1 small, possibly remote, AI's city captured - is not that much of a reputation hit; and, faith allowing, you can buy a few missionaties of his religion right away and then you can even gift the city back to its owner, if you'd want to; OR
- remove inqusitors from your cities - move them 3+ tiles away, and allow AIs to convert your cities to their religion. Works best if your neighbour is one with the religion which allows religious building(s); OR
- puppet/annex some cities with desired religion, get religious building(s), then convert 'em to your own religion. Works best if you're playing wide and warmongering.
To do p.3, either
- spread your own religion to your city using a great prophet or missionary of your own religion. Works best for small and new cities. If using missionary and it's a brand new city you create, then you may want to halt its growth at 1 until your missionary converts it back to your own religion; and you also might want to "weaken" the missionary you use in p.1 (foreign religion's one) by putting it into some foreign territory, 1st (to 750 religious strength). This will make your own missionary, being at full strength (1000), to make your city back to majority of your own religion in one use, saving the 2nd use for another time. OR,
- use your own religion's inquisitor to remove all religions but your own. This works best for larger and older cities which still have some followers of your own religion, but too few of them for a single missionary to convert the city back to your religion's majority. Single use destroys the inquisitor, but for large cities, the work he does may be worth more than many missionaries' efforts put together.
In my last game, i had Celts going Catholicism, taking followers' beliefs for Pagodas and Monasteries. Well guess what, 4 out of my 10 cities have both buildings, 3 of them were getting 'em when i built those cities - so from the start, these had massive boost to culture and faith AS WELL as 2 other follower beliefs being effective (my own ones), since i converted them back to my own religion very next turn after getting those buildings. Nice!
P.S. All the above, plus the fact that AI's _love_ getting beliefs which allow to build religions buildings, - are reasons why i usually avoid getting beliefs for religious buildings myself. If i would not be as lazy as i am, then i possibly could have majority of my cities having all religions buildings despite going without such beliefs, and without keeping my cities' majority religion being something different from my own religion (more than for 1 turn that is). ;·)