Regrettably, Mr. Trudeau has ruled out a coalition with the NDP. I think that's another one of his bad, rookie mistakes. I believe that takes the formation of a coalition after the election off the table.
If Mr. Harper were to get the most seats, but a minority, and the Liberals and NDP then tried to form a coalition, Harper would be completely justified in claiming that voters were mis-led. In that situation, I don't think that the Governor General would ask such a coalition to form a government. The GG would have little choice but to opt for a new election.