How come this guy got credited a song before Ferre Gola? I would have been pissed lmao plus the kid doesn't even speakblingala in the entire song, was he even Congolese?

It’s world Music ?

for the simple reason that werra likes doing something different so a kid like him singing pop music so outside him werra tata masela and chef christian mabanga he had to throw in the kid
the way for example in sous sol, petit lokua l enfant mystere got a song before even heritier

Maybe it has already been explained but why Heritier didn't have a song in A la queue leu leu's album while he was at recording sessions ?

Because he was a new member. Those who had since where bandmembers who were there since day 1 (Baby Ndombe, Ferre Gola, JDT Mulopwe, etc.) + de admins.