I am by no means a constitutional expert but I'm at a loss to understand just what Mr. Harper could do if the GG asked a coalition to form the government.
This isn't the same thing as his vow to 'go to the people' during the 2008 prorogue. His party was the government at that time and and there was no question that he was prime minister acting, skulduggerously as it was, within his powers.
Could he go the Supreme Court? My feeling is that unless he's bribed all the justices, he'd be sent packing.
What then could he do? Refuse to vacate 24 Sussex? Call for armed insurrection from his supporters? Refuse to attend parliament?
Could he attempt to jump the GG and go the Queen? Should the GG grant the other parties the government, Harper would not be Prime Minister and would thus, as I see it, have no authority to ask the Queen to remove a Governor General he didn't like or instruct that GG to revisit his decision. I'm not sure Ma'am would have anything to say to a petulant ex prime minister other then to send him back the GG he appointed. Historical precedent suggests the Monarch has no interest in playing an active role in domestic constitutional problems:
I am commanded by The Queen to acknowledge your letter of 12th November about the recent political events in Australia. You ask that The Queen should act to restore Mr. Whitlam to office as Prime Minister.As we understand the situation here, the Australian Constitution firmly places the prerogative powers of the Crown in the hands of the Governor-General as the representative of the Queen of Australia. The only person competent to commission an Australian Prime Minister is the Governor-General, and The Queen has no part in the decisions which the Governor-General must take in accordance with the Constitution. Her Majesty, as Queen of Australia, is watching events in Canberra with close interest and attention, but it would not be proper for her to intervene in person in matters which are so clearly placed within the jurisdiction of the Governor-General by the Constitution Act.I understand that you have been good enough to send a copy of your letter to the Governor-General so I am writing to His Excellency to say that the text of your letter has been received her in London and has been laid before the The Queen.I am sending a copy of this letter to the Governor-General.17 November 1975
So candidate Harper can make all the suggestive and subversive statements he wants about how the legitimacy of coalitions and the like are open to debate. However, like his views on the contempt ruling from the Speaker that started this election, he is voicing nothing more than his blinkered feelings. My guess is that these statements are all smoke and mirrors for his idiot base to make them think he's got some sort of magical power to overturn the final result of a federal election. The reality is that despite the the endless monotone "let me be clears", Harper is a weak man in about the weakest possible position he can be in as a party leader in a federal election.
I'm also quite sure an academic like David Johnson has done his homework on the issue and is watching this election very closely.
UPDATE: The specific question asked of Harper was:
"Will you or will you not accept a decision by the Governor General to call on the second party to form a government if you again lose the confidence of the House?"