Given that there is no PR RO compliance issue, the situation is fairly simple, as I posted before:
If the rule for PRs is being enforced for the particular individual seeking to board a flight to Canada, that requires the PR to present either a valid PR card or a PR Travel Document.
If the rule for PRs is not enforced for the particular individual seeking to board a flight to Canada, a visa-exempt passport will suffice.
In my previous post I linked the webpage with the same rule you quote. That is the rule. (Admonition at top of IRCC webpage: http://www.cic.gc.ca/english/newcomers/about-pr.asp )
There are differing opinions about what the odds are the rule will or will not be enforced during the leniency period. Whatever one thinks the odds are, or why, there is no guarantee, however, no formal assurance that the rule will not be enforced during the eTA leniency period.
The sparsity of reports since March 15 (just three that I have seen), of PRs attempting to fly to Canada without a PR card, suggests to me that very few are electing to take a chance on the rule not being enforced. But in contrast there are some who express confidence the rule is not likely to be enforced.
In this case the risk is mere inconvenience. Worst case scenario, if she encounters enforcement of the rule, is having to obtain a PR Travel Document to make the flight. Or, she could go ahead and obtain a PR Travel Document just to be sure . . . if she travels after the leniency period, she will for sure need a PR Travel Document (or do the travel via the U.S. route).
By the way: PRs are Canadians. Canadians have a right of entry into Canada. All the PR has to do, once the PR reaches a PoE, is establish identity and status. Any valid passport plus an expired PR card or CoPR easily does that. Even though it would likely invoke probing questions and a severe admonition about carrying proper documents, even just a Canadian drivers license should suffice.
The problem, if there is one, is usually in physically getting to the PoE. Requirements for boarding planes are actually more strict for PRs than what is required at the PoE itself. In the meantime, the U.S. imposes its own somewhat strict requirements for travelers, so traveling via the U.S. is not always a convenient way to physically get to the border.
Another by the way; re visa-exempt visitors and return trip tickets:
The issue about having a return trip ticket has to do with appearing to legitimately plan to visit Canada.
For a PR pretending to be a visa-exempt Foreign National traveling to visit Canada, the return trip ticket just helps (allegedly, I doubt its relevance these days) to satisfy the airline the traveler is visiting Canada and entitled to board a flight by displaying a visa-exempt passport. Once this individual reaches the PoE, a return ticket is not going to mean anything.
But for an actual Foreign National (that is, not a PR) with a visa-exempt passport, upon arrival at the PoE in Canada the examining officers might be suspicious that a traveler without a return trip ticket might be planning to stay or work in Canada, and thus is not genuinely just visiting, and thus not admissible as a visitor.
Remember, all a visa-exempt passport means is that the individual has authorization to enter Canada without obtaining a visa in advance. It does not constitute actual permission to enter Canada.
At the PoE it does not matter so much whether it is in an airport, for those who just landed on a flight originating abroad, or at a PoE on the Canadian side of the border with the U.S. A traveler with a visa-exempt passport must present herself or himself to the border officer at the PIL for examination prior to entering Canada. Actual permission to enter is not guaranteed (for a Foreign National; PR is entitled to enter). And if a visa-exempt passenger gets off a plane, has no return ticket, and upon being asked questions is vague about how long she or he plans to be in Canada, odds are high the questions will get more probing. The answer "six months," is bound to make matters interesting, at which point what the traveler will be doing in Canada, where, why, and of course what funds the traveler has, are all questions which are likely to be asked. If the examining officer is not satisfied the traveler will not work or overstay or become a financial burden, the officer can outright deny entry; or the officer may allow the traveler to enter for a specific amount of time. (In the course of my life, before becoming a PR -- I am now a citizen -- I had been restricted to as few as two days in Canada (after landing in Montreal on a flight from Europe), and other times issued a Visitor's Record for various periods of time, with must leave by dates ranging from a few weeks, to a couple months, and once one for six months (while I had a PR application pending). But on more than a hundred other occasions I was simply waived into Canada and on those occasions I could legally remain in Canada for up to six months. I personally have experienced that how things go for dozens and dozens of times does not dictate how they will go the next time, even when all the relevant circumstances seem to be much the same.)
The six months is merely the maximum allowed stay when a FN is allowed to enter Canada and the duration of the stay is not otherwise specified by the border officer. Thus, for those casually waived into Canada, technically they can indeed stay six months (and among those hundred plus times I was causally waived into Canada, I pushed this envelope on several occasions, which on subsequent trips coming into Canada led, as I noted above, to being restricted to much shorter stays on a couple occasions).
