I was erring on the side of caution yesterday but having another look at your pics. this morning, I’ve got to say it looks appalling. It should not have cracked already either & there should be no give in it; you won’t be able to fill the crack, it’ll only reappear without more radical reinforcing.
I’d be inclined to take it all down & start again but this time level it properly, reboard & reskim.
I’d be inclined to take it all down & start again but this time level it properly, reboard & reskim.