The nuance here is that the Governor General by convention follows the advice of the PM, who establishes their continuity plan at the start of their government

The nuance here is that the Governor General by convention follows the advice of the PM, who establishes their continuity plan at the start of their government (IIRC it’s a standard party document and they just update the names). It’d cause a crisis with the monarchy but they’re not legally bound to appoint the deputy. Like you said it only applies to special circumstances, I was just addressing the legal mechanism by which it happens (as the OP talked about a PM dropping dead).