She could just say no

As this completely unprecedented, there is a possibility, albeit remote, that the Governor General opt to do “none of the above.”

Constitutionally, her power is not defined. To that end, it is possible that she denies the proposed coalition to form government and refuses to issue the write for election. She could simply refuse to accept Mr. Harper’s resignation. This could create even more instability, but allow for the Government to remain in power given that there is no precedent for a coalition of this nature.