A formal mechanism doesn't exist because it wouldn't be seen as democratic.
A similar scenario happened in Manitoba, where Selinger had the voters (well, NDP members) choose who to be leader. I think this is what Canadians would expect. Either the leader sees the writing on the wall and resigns, or puts it up to a vote of the membership (not the MPs/MLAs)
You are forgetting that in Canada party leaders have been toppled at party conventions when there has been a leadership review that didn't pass some imaginary threshold. In 1982 Joe Clark was toppled as PC leader when he only got 67% of his one party to support him. He then ran to succeed himself and lost. Similarly Ralph Klein quit as Premier and Alberta PC leader after he only got a 55% vote of confidence in a leadership review.
