Were you wrong to have "mentioned her as a previous common-law partner"? Probably not, since by the time you did so, you were aware of the IRCC 1-year rule and you were striving to be forthcoming.
I would underline here a strong yes that it was correct to disclose this relationship as common law - because yes, ircc DOES catch people having lived together for more than a year (in subsequent apps) and treat these as undisclosed relationships and enforce bans. We've seen multiple cases of eg students in shared student housing where there's little/no evidence of relationship at beginning of the shared residence (because yes that is how people meet or end up in relationships). Some of these get worked out, perhaps most, don't know - but it's a hassle and a risk.
So for anyone else reading: if in doubt, disclose. Shared addresses are the first thing they will look at.
Side note that residing together is not sufficient - and so we have the asymmetric approach of IRCC that any evidence of residing together can be used against you, but is not automatically sufficient to show you were common law - they can and will ask for considerable supporting evidence of having a 'joint household' (expenses, purchases together, etc). And vice versa.
I mention this point - the asymmetry - purposefully to note that they do NOT see it as their role to be neutral in application of these concepts, but rather primarily in context of 'protecting Canada' and integrity of immigration system etc (as they see it, not as outsiders might).
At least, that's how it looks to me and I cannot imagine the IRCC arguing that you were common law throughout and you'll be penalized for a failure to so declare. Frankly, I would find that a tad outrageous. Heck, were that to happen, I might even get my barrister's robes out of mothballs and take your case on judicial review pro bono. It would have to be pro bono, since I have never done an immigration or any Federal Court J/R and I'd probably make a hash of it, but it would be fun to try.
After the additional information, I would overall agree, but not sure that I would be that surprised or consider it 'outrageous' if IRCC were to consider differently or - perhaps more likely - ask for further information. Why not outrageous or surprised? Because it is - admittedly - a subjective interpretation (in either way), without the obvious, reliable, and unambiguous evidentiary basis that an outsider could use to infer intent. Meaning, simply, all anyone is mostly going to have is the 'story' told by the sponsor and applicant.
That said, a few minor thoughts (note, not suggesting anywhere here being untruthful, but you can be your own advocates and present carefully):
-not having seen each other physically for two years or during the period - where originally intended or not (i.e. the pandemic issue) is a really strong point.
-you were exclusive is not a strong point, but you can choose what to emphasize - i.e. those other dates (at family encouragement) do actually count. (pandemic making any kind of dating a challenge is a related point)
-your communication 'weekly if not daily' is in between. Choose your words carefully. I'd say weekly being the base agreed contact frequency is not strong indication of a permanent relationship.
-a thing to think about how to include: common law implies a shared 'household' - that even if residing apart, must continue in some way. Whether that's shared expenses or 'stuff' or responsibilities or ongoing things like shared friends/acquaintances/family. If I get the relationship more accurately this time, and since both sponsor and applicant returned to their respective and separate home countries after, it sounds to me like there was very little or nothing that fits this description (if there was, perhaps only some shared school and/or work friends).
Also it sounds like there was clearly no agreement to eventually actually live together nor decision made on where, and certainly not when. It wasn't excluded, but not a plan.
A different way to think of and frame this: if sponsor and applicant at some point during this period had gone their separate ways, would there be anything at all to 'divide up'? If not, sounds to me like a real split. Find some way to weave these points in.
-as my belief/suggestion is that a letter of explanation (to which you both agree) is advisable, think about how to to make these points - that while you remained in touch and somewhat romantically involved, there absolutely was not a 'continued and continuing household' / relationship that could be considered common law of any sort in this period in between.
Good luck. Hopefully this will go smoothly. Despite my caution, I'm not at all predicting that this WILL be a problem, rather warning that it's not impossible and would/could be very unpleasant if it became one.
