How do you change the plot-culture readout to display the leader's name rather than the culture's name? (ie, instead of 70% English/25% Maya/5% Khmer, 70% Elizabeth/25% Pakal/5% Suryavarman)

When you set a civ's names, ie, Leadername, Civ description, Short description, Adjective
for example:

Bismarck
German Empire
Germany
German <------ this one, the adjective is used for that.

so if you change the adjective to be the leader name that'll do what you want.

unfortunately, you may get things like "A farm has been destroyed by a marauding Huana Capac Swordsman" instead of Incan Swordsman

Aside from changing the code in the SDK, there's not much to be done.

When you set a civ's names, ie, Leadername, Civ description, Short description, Adjective
for example:

Bismarck
German Empire
Germany
German <------ this one, the adjective is used for that.

so if you change the adjective to be the leader name that'll do what you want.

unfortunately, you may get things like "A farm has been destroyed by a marauding Huana Capac Swordsman" instead of Incan Swordsman

*blink* I hadn't even thought of doing it that way! And actually, that'd be perfect, as the entire reason I wanna change it is because there will be several distinct "civilizations" which will actually be different leaders of the same civilization vying for power...

oh, well perfect then, happy to be of service.