Alright, fairly straightforward, but possibly complicated, question: What is the minimum amount of changes required to make a new civ? For example, if I wanted to create a civ that was identical to the Amurites, but was called the "Magey-Peoples" (and still leave the Amurites untouched), what would I have to do?

I'm asking here because I assume someone already has a pretty decent idea and asking would be faster than manually looking through every XML/py file to see what references civs.

I know you would have to change/copy portions of C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Beyond the Sword\Mods\Fall from Heaven 2 030\Assets\XML\Civilizations\CIV4CivilizationInfos .xml. If you want to give them a new leader you would need to add him/her to CIV4LeaderHeads.xml in the same file. I'm not sure if you'll need to alter any other files.

Will the newly added civ show up properly in all menus/everything without further work?

Nevermind, I'll just try what you suggest and see what happens...

Most of the stuff (UUs, UB, leaders) will show up automatically, but none of the textkeys would be automatically added; unless you change them, your new civ will appear to be the same as the civ you copied (the name, strategy tips, etc). You can either write what you want them to be called (etc) in the place of the textkeys, or create new textkeys in C:\Program Files\Firaxis Games\Sid Meier's Civilization 4\Beyond the Sword\Mods\Fall from Heaven 2 030\Assets\XML\Text\CIV4Game_FFH2.xml

I'm eager to see how this goes since there are plans for 2 lizzie civs to be added. Non-official additions, that is.

Does modular XML loading work with FFH?

I've heard it does; Sureshot seems to have a mod that makes use of it. No modular python, however.

Yes, Modular XML works, but only if the Module was written for FfH. Using things that work on BtS utterly fails to work on FfH because so many variables are missing from any file you attempt to alter.

The stakes for adding a new Civ have been raised in 0.30 by the addition of world spells.

Nah, that part isn't so bad. WorldSpells are just a spell that is restricted to a single Civ and tagged as Global. So you could simply not give them one and it'd be fine.

If you do choose to give them one then you will have to go into Python to set up how it works just like any other Spell, but you could be boring and "copy/past/minor edit" some other spell