As to who becomes a party leader: Party leaders are picked internally by party members via leadership races, not by the general public. This is good because the leader is selected not by "Low Information Voters" swayed by short-term issues like egg prices (as in the US), but by people who have gone through a qualification filter.
Post-election, the PM must sustain the House's confidence, with no-confidence votes possible anytime. So it is not necessary to watch helplessly for 4 years while your PM destroys the country.