Sorry, I'm not an expert in Canadian immigration but how can you be held up in immigration when origin is Canada and departing Canada on a US bound flight?

Anyway, for such travel disruption on the day of departure (IRROPS) I would have expected CA ground staff to take responsibility and do the rebooking regardless that it is LM award ticket. It seems that the CA agents took the easy way out; "contact the issuer for the ticket". By no means not the first time (nor the latter) that a carrier will try to weasel out this way when seeing that it is an award ticket issued by another carrier/FFP. Did you call CA rather than speaking with CA ground staff?

Now you are in the mercy of LM and LM is likely to view this a voluntary rebooking (and as you have already learned may deny such rebooking due to first segment already used) unless the agent understands that rebooking is needed due to IRROPS.

Good luck!