Coates reversed the "tchalla aint king anymore" thing the moment his second arc started.
It is a constitutional monarchy based on a few panels btu all the new "parliament" people did is beg T'challa to do everything for him.
He seems to have full autonomy to do what he wants. And even if he didn't and the parliament had more power... Shuri and Ramonda aren't the parliament. T'challa still outranks them.
Basically Coates spent a year trying to change the wakanda government and then the movie came out and marvel editors said "naw brah, and by the way, make sure to suddenly bring Okoye back to life and make sure the doras who hate you now like you again and do that movie salute thing and oh btw, all that work you spent making Shuri Griot... naw she a super inventor genius now that shits got to go"