Most colonialists had self-interest first in what they had to do with their territories but that does not mean it can’t be corrected when they leave, in the case of Nigeria –Amalgamation being the issue. Countries are united by these factors – Language, Religion, Custom/Culture, which unfortunately is not the case with Nigeria.
Federalism which is unity in diversity is very important in our present situation but the success would still depend on having at least one of factors mentioned above. You said Indonesia is a unitary country that is developing but failed to mention that it is the country with the largest number of Muslims in the world meaning they are united in religion. Though they had Mohamed Suharto who was a corrupt leader and dictator, most of his stolen wealth remained in the country, meaning he thought of his country first though he was doing something wrong. Compare that to looters in Nigeria who never think of developing any business in Nigeria
The Old Midwest /Bendel state was a miniature Nigeria but was one of the most united states because there was a common language (broken English) replacing differences in dialects and everybody thought of Bendel first with slogan like “Up Bedel”. Compare that to Nigeria where we think of tribe, local government and state before country and the further you go the love and unity reduces.
The only thing uniting Nigeria presently is corruption and at times football when we are winning, when we lose, you hear people saying it is the fault of that person who is not from our place, forgetting as a team you win and lose together.
Nigeria is being forced to be together through amalgamation as most of the old Easter European countries were being forced to be together through dictatorship and we all know what happened. Sincere federalism will be the only way to “manage” this contraption called amalgamated Nigeria