Based on the way he vilified the concept when it threatened to take away his minority, through the confidence votes he was forcing, Harper is terrified of a left-centre coalition. Harper is an "us vs them" kind of person so he riles on dividing his perceived foes. A democratic, mature, constitution-based coalition could stand a chance of reducing him to a minority without confidence that could be replaced by the coalition.The candidate most likely to defeat Harper in each riding runs for their party "as supported by" the other parties. The leader of the party of the coalition with the most members who won seats could be PM. The others would be cabinet members.
2-party systems try to squeeze at least twelve viewpoints into a dichotomy: fiscal, social, political and environmental liberals, conservatives, and centrists.
Coalitions, like proportional representation, is far more democratic than what we have now. because it requires negotiation, compromise, and cooperation. Proportional representation would strongly encourage coalition governments, but we work with what we have as we work for something better.