I'm useless at guessing age so I'm always relieved other people also find it difficult.

I agree that each tribe absolutely would have challenged if they hadn't respected T'Challa, but I meant ceremonial more in the sense of that it was known that T'Challa is respected by the tribes and it would be a big surprise if they'd suddenly declared otherwise. T'Challa has been training to be a person the tribes respect all his life, and he should have had a good sense of whether or not the tribes would challenge in advance.
(If the kingship went to a different tribe, I wonder which tribe the royal family would belong to. Ramonda's?)