So my question was: were you saying that the RCC's position is (something like) P, or that the RCC's position is (something like) Q?

Something almost exactly like Q'.

But it's possible, as you say, that actually the position being sketched in HG would be, if written out with more care, something more like this: there are essential dogmas of RC faith, for which one must assign p=1; there are ordinary statements of fact, like (ha!) "the earth orbits the sun", for which one would be ill-advised to assign p=1 but the RCC doesn't take any particular position on that question; but for evolution the RCC specifically says not to take p=1 but leaves open the possibility of taking p=1-10^-20 or something.

I get the impression they were hinting at something more like Pr(evolution)=0.9, which is a figure entirely unsupported by anything in the text and involves me taking a guess, but apart from that this is pretty much exactly how I read it, yes. (With the note that the only reason the RCC is specifically calling out evolution is because there's been such a brouhaha over it from the protestant churches that staying silent on the matter would have been bad politics).

senior RC clergy don't generally think in terms that correspond so directly to the very quantitative probabilistic approach commonly taken here as to enable a clear distinction between p=1 and p=1-10^-20, etc.

While there may very well be senior clergy who do think in such terms (I wouldn't know, I've never met any seriously senior clergy) this is largely why I think that something like p=0.9 is probably closer to the intended reading. (Or p=0.95, or even p=0.99)

(E.g., p=1, as such, means that absolutely no possible evidence would change your mind, but I think Pius XII could probably have imagined possible happenings that would have convinced him that God doesn't after all directly attach souls to human bodies.)

Even if so, I'm pretty sure that it wouldn't be the official Vatican position that that probability can ever be anything less than one. (Politics, again; if the Pope ever admits to that p being less than one and someone runs a headline based on it...)

And if he does see such evidence, he has to consider the possibility that he is hallucinating, or being intentionally tricked by someone; if p is high enough, then it may very well be the case that any sufficiently convincing evidence will merely convince him to report to the nearest psychologist with complaints of extraordinarily detailed and persistent hallucinations.