Belgium has three official languages: French, Dutch and German. Nowadays, the official language of the Democratic Republic of the Congo, whose territory encompasses most of the Belgian Congo, is French. Is it because Belgium favored the French language? More generally, what was the language of the administration in Belgian Congo? And of the European population? Which languages were taught at school?
Even though i can’t give an answer on all the questions you gave, I’m going to try my best to give you the best explanation on why french is currently the main language in the Democratic Republic of the Congo
During the Belgian colonisation of Congo, french was the prestige language in the country and it was spoken by the people living in the Walloon region, the royal family and all the upper classes, even though dutch (or flemish, which is the name of the local variety of dutch in belgium) had about the same amount of speakers (or even more).
The reason for why french was considered the prestige language go way back to the Belgian revolution and the founding of the country, back when the country was part of the Kingdom of the Netherlands. French had always been the main language in the administration of the south of the country at the time plus back then, french was the lingua franca of europe.
So when king Leopold II went to colonise the congo (then called Congo Free-state) and claimed it as his own private property, the language used in the administration of the colony was french. I also want to point out that the french colonies in africa were nearby so that could’ve also been a factor in making the language more widespread.
As for the languages that were taught in the colony, the Belgian Congo was one of the only colonies in were the local languages were used to communicate and some of those languages, like Lingala, were even thought in some schools.