This phrase strains credulity (or even credibility)

An article in today's paper, reporting a lawyer's contention that it "strains credulity" that Stephen Harper was not aware of his senior aides' plans, got me thinking about that commonly-used phrase. This is not so much one my pet peeves, like the hijacked misuse of words such as momentarily, alternate, etc (see a previous blog post), but a genuine confusion, because I do think that both "strains credibility" and "strains credulity" COULD be correct under some circumstances. But, generally speaking, I think that "strains credibility", as in this case, is more likely to be what is actually meant.
Credibility means believability, an ability or a worthiness to be believed or trusted. It can be used of both people and things, and is usually a positive quality. It was introduced into English in the late 16th Century from the French, and ultimately from the Medieval Latin credibilitas, meaning believability.
Credulity, on the other the hand, means gullibility or naïveté, an excessive readiness or willingness to believe. It is always used of a person, not a thing, and is usually used with a slightly negative connotation to refer to a person who is easily tricked or taken advantage of. It was introduced into English from the French in the early 15th century, and originally derives from the Latin credulitatem, meaning easiness of belief or rash confidence. According to Garner's Modern American Usage, credulity did originally mean belief, but that usage has long been obsolete.
Just for good measure, another word often confused with one or the other (or both) of these is creditability, which means a worthiness of praise, honour, commendation or credit. It is much less often used, although the adjective creditable (worthy of praise) is quite commonly used.
Thus, the phrase "strains credibility" refers to a person, situation or assertion that pushes believableness beyond normal bounds i.e. it is so bizarre or excessive that it is difficult to believe it can possibly be true. The phrase "strains credulity" could be taken to mean that something is beyond the powers of even a credulous person to believe, and so, as a phrase in its own right, it may not be definitively wrong. Given that credulous people tend to believe almost anything without thinking or "straining", this is perhaps not as effective or strong a phrase as "strains credibility", as Paul Brians' "Common Errors in English Usage" suggests, although it could be just as "credibly" interpreted as a stronger phrase, meaning that something is so unbelievable that even a credulous person would not believe it.
"Strains credulity", though, seems to be in the ascendancy in print and on screen, even though, in most cases, I think "strains credibility" is what is really meant. The Dictionary of Modern Legal Usage, for example, counsels against using it, as does Garner's Modern American Usage, although I have come across many uses of "strains credulity" in judges' summings-up and other legal statements. A couple of examples I have encountered in quite "credible" publications (here and here) appear to use "strains credulity" as a stronger version of "strains credibility", in the sense of something that not only "strains credibility" but even "strains credulity". However, I have also seen another article where "strains credibility" is CORRECTED (incorrectly, in my opinion) to "strains credulity".
The verdict? The jury is still out, although I still believe that most uses of "strains credulity" are not well-considered, and "strains credibility" is what is actually intended.