Partner availability in J
 

Seeking to book a DCA-YUL-YVR trip using United miles on AC metal. The first leg shows J availability but the second does not. This is for a trip in about 2 weeks. I can see from an AC search that there are open seats in J on that second leg, though not a ton (looks like 8 of the 32 J seats are selectable with the rest occupied). Can I expect them to be released for booking via United at some point?

No. Every seat is bookable with Aeroplan points. Not through partners.

So only a portion are made available to partners and that portion does not increase as the flight date approaches?

This is true of nearly every airline in every alliance, for example it absolutely applies to anyone trying to book United flights using Aeroplan miles.

There's no single answer to this. Sometimes they'll open new partner award space if a flight is selling weakly, or if a route sells so well that new flights get added to it. Again, this is true for most airlines.

If there's a set policy driving that decision process, other than "revenue management determines on a case by case basis" it's not published to my knowledge.
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Unlikely. Just like United flights are not available to Arroplan redemptions

Empty Seats
 

I just flew on AC33 YVR-SYD. This 777 had 16 empty seats in business. Who can explain this to make us feel better?