That’s interesting that CBSA would let someone into the country by mere photocopies of documents and foreign passport.
Repeat: for someone who is in CBSA/IRCC database (which means, basically, everyone who has ever applied for PR status, received citizenship/passport, and several other categories),
they have the info. Big exceptions are mostly those born in Canada who have never outright applied for some things, and Canadians born abroad who've not applied. Hopefully in most cases these are children but any way not relevant to this case.
They don't even need all these documents, but they do need
some. To identify them sufficiently to match - and of course to convince cbsa officer that Person A in their file is also the Person holding the identification of Person A. (Everyone faces that issue, hence eg biometrics and pictures.)
Match issues between the database and docs: sounds silly but mostly name and date of birth, supported a bit by other stuff. UCI helps a lot. I presume there is other stuff in the files but don't know the details (they probably do record the foreign passports and probably some other docs that they see and link them to the file, but again - don't know). So yeah, change your name and not have docs on that / update your other docs, they might not match the file.
I have
heard of citizens being admitted with no 'real' docs - but were cases of stolen docs and the like, and I think there were ways to get photocopies or something sent. These were not described to me as fun encounters at the border - but then, replacing a stolen passport while abroad is not a lot of fun any way you have to do it, either.
But admitting Canadians who have other supporting docs and just lack the passport or even citizenship cert? Relatively routine. (That might still mean extra time at the border for them to check, and still not be 'fun', but most cases I've heard of have been relatively mild encounters, with the big difference in time only if there are lines at secondary inspection (if sent to secondary). And have heard of quite a few cases where the CBSA officers really only asked a few more questions and wasn't all that much esxtra time.
And I dont' offhand recall hearing of a straightofrward case where the citizen was not admitted.