"are one motion from the House of Common from being overthrown." - so this is a form of political constraint, which we can see in the US doesn't work very well if the ruling party wants to ignore concerns and acts at the behest of the Executive.

If the PM holds enough popular support and has even a narrow majority that he can effectively whip, he's almost above reproach.

Everything at the top in Canada is 'convention' even the Constitution and there's barely any real constraint at someone driving a truck through all of it.

Yes but that's marginal because support is entirely contingent on whether the legislative branch members believe that support won't get them voted out.

The US executive is very different because it's an independent election: it's almost impossible to get rid of a President, and relatively easy to deflect blame.

Australia's round of axing prime ministers had some essential logic to it despite the move being relatively unpopular with the electorate: it wasn't about whether the party would lose power, it was about whether replacing the prime minister would let them retain seats they faced otherwise losing.

It's a mammoth difference when the election for executive power and legislative power are linked and it shows.