give it a try and call Topbonus.
you will have the best chances when you verify availability beforehand and give them flight number one after the other and the date of your desired award.
Be aware that Topbonus has a rule to require the direct route to avoid backtracking to Canada f.e. in your case. The rule is however not consistently applied by agents.

Also it will not be an AA award but a mixed Oneworld award which might have different rules or might require a higher mileage than the 15k for a simple North American AA-award.
Stopovers are not allowed on partner-awards, so the maximum stay at YVR and JFK is 24 hours.
A land connection between SEA/YVR is not possible.
2 connections should be ok, but Iīm not aware of the detailed rules, which might limit the connection to just 1.

there are a couple of reasons why you might not be able to book the award for 15k miles, so donīt be dissapointed if it does not work. Award rules are not very transparent on ABīs website, especially using 2 partner airlines on 1 award is a black box (at least for me).