On this important question (does time spent abroad accompanying your Canadian spouse count toward your 730-day quota) , I consulted PMM again, and spent a delightful half-day pouring through the Immigration Act, and a few court cases that dealt with this issue. Thanks to all those who pointed me in the right direction.
The short answer is that yes, time spent abroad DOES count toward the quota -- PROVIDED THAT it is the spouse who "accompanies" the Canadian, and not vice-versa. Much depends on the definition of "accompany", which means "be with" the other person. According to the court decisions, the Canadian cannot merely be abroad to be with the spouse (or else the court considered that it was the Canadian who was accompanying the PR, and so the days did not count. If the Canadian has another good reason to be abroad, such as a job or enterprise, then the couple has a stronger case to say it is the PR who is accompanying the Canadian.
And (I surmise) it helps the case if the PR does not have a strong reason to be in that particular place -- except to be with her Canadian spouse.
If all this seems a bit too complex for a border guard to judge on the spot, it probably is. So the moral is to play it safe, try to amass the required 730 days IN CANADA (so there's no argument or doubt), and rely on the provision for time abroad ONLY if absilutely necessary.