So in theory my UK spouse could do a soft landing on his own, before me, with me still in the UK to wind things down, as long as he stayed for 15 days and followed quarantine rules, as they might apply when he gets CoPR?
As I understand it, yes. But I'm not 100% certain; the intention obviously is to join a Canadian citizen / resident who are actually resident. But I don't think they are attempting to fully check physical presence of the Canadian (incl PR) partner, nor fussed about the timing like in your hypothetical test. They just want to confirm you meet the exemption test - that is, spouse who is Canadian citizen or PR - address and isolation plan, etc. I've certianly not heard anyone claim there was an issue nor even asked about this. And I doubt they'd turn away someone who said their spouse was unavoidably detained but coming in a week. No guarantees, however.
Note key point though: there are two steps - getting on airline and being let in the country by CBSA. Airlines seem to want only evidence of eg marriage to a Canadian; CBSA - right to entry.
That said: it would obviously be preferable to travel together. And possible we've never heard of this because nobody's tried, and also, CBSA can change their mind about stuff.