Sorry for the initially incorrect title.

For some reason, I cannot upload screenshots nor post links (maybe because I'm a new user?), however the CA$1199 fare for YYZ-KEF is available on most days January to April. For onwards connections, the fares start at $CA1299. If you go to Icelandair's website and select Canada, you will see the offer page.

For SEA, the lead price to Iceland is US$1099, available on select few dates also between January and April (e.g. 5-13 February), and US$999 for most onward connections (e.g. to AMS, DUB, Scandinavia), for which the availability is a lot more.

Regarding DTW, the lead fare is available on about 90% of the days on flights to Iceland, and about 50% of the days for onward connections.

My figures above are based on 8-day trips using the price graph on Google Flights.