Also 2 J seats from PHX on the 28th or JFK on the 29th.

The flight you want is looking quite open in J and WT+: J/9 C/9 D/9 R/9 I/7 W/9 E/9 T/9. I wouldn't have thought there would be a lot of premium last-minute demand over the holidays. So I would think there is a decent chance either WT+ or J would open up for you, but judging by the current near-term availability, probably not until mid-December.

Personally I would book the WT+ flight from SEA and then wait a couple of weeks to see what happens, hoping something better opens up. Worst case you have to buy a £200 connection to SEA. Also keep your eye on ORD (particularly in F, which costs less miles than YYC: 60k in F vs 50k in J from YYC) and YVR which have low loads and seem to open up two weeks out, and cheap connections (particularly if you have some Star Alliance miles).