Your assumption about the motives of the British and French after WWI is totally mistaken. These colonialist countries absolutely took into consideration ethnic and religious divides. Their whole purpose was to compose countries with such a mix of incompatible entities that they would be easy to rule. Remember the old “divide and rule” saying. They made sure there was lots of division. They also did it in Africa, for example Nigeria with Yoruba, Hausa, and Ibo. You may not remember the vicious Ibo war a few decades ago when the more competent Ibos wanted their independence or at least autonomy from the rest of Nigeria. A hundred years ago, the only fubar was the fact that these diplomats represented the “deep state” of their era and deliberately made these messes.
Your point about Nigeria is demonstrated again by current events - the Government is conducting another genocidal campaign against largely Christian ethnic groups who were shoe-horned into Nigeria when formed, and who have been bucking to get out every since.
They didn’t take into consideration ethnic and religious divides in considering what’s best for the locals. The referendum, as I said, is a correction on that.