The idea of the Conservatives voting to keep Trudeau in power is absurd. The Liberals would either have to do a deal with the NDP or a new election would be forced after each party fails to get confidence. The Conservatives would be very happy to rush both the NDP and Liberals back into an eleciton should those two parties fail to work together. The Conservatives have absolutely no incentive to work with the Liberals and no incentive to have any party other than their own obtain confidence. The Conservatives are in the best position to go through instability and 2-3 elections in quick succession. The result, they believe, would be the near bankruptcy of their opposition. (I think it may not be as serious as Caniadans may become less susceptable to politcal advertising given the excesses we have seen.)
As much as you may hate the Liberals it is clear that they would have to work with the NDP after the election if there is no majority -- out of very obvious political necessity. Mulcair, for his part, has recognized this. He too would become a reluctant but willing partner if the Liberals had more seats. Given these circumstances it would be a game of chicken. Both parties aware taht the other party needs them hoping for a good hand in bargaining but each also aware that the need would be mutual.
Anyone who does not think the parties would have to work together should take a trip outside the political echo chamber and listen to what people on the street think. You would find that there is no debate. Canadians presume that the NDP and Liberals would work together. If you suggest that they might not, the result would be anger. Liberals would say it would be the NDP not their party that woudl refuse to do a deal while NDP supporters would say it is the reverse. Both would agree to punish whichever party would be so stupid as to leave Harper in power. And trust me they would.
I disagree that Harper only needs one of the two parties to conceed. Conceeding defeat is not a promise of confidence. Harper would need both parties to conceed to have a green light to continue. If one party conceeded and the other did not that party would still have the option to discuss and possibly support the one that did not conceed. If both were close in seats then either party could propose a government. In the light of that proposal the concession would be void.