Just say you clear the EUA when your window opens up, you'll be assigned a single seat all the way through. Yes - you'll still be seating at your assigned seat when you reboard the plane at SEA.
As for the second part of your question, the CO Revenue Management defines the x number of FC seats designated for A and F classes, for pax traveling from IAH to ANC. At the same time, they'll also assigned y number of seats available for A and F classes, for passengers originating from SEA to ANC.
If you take a look at ITN.net, you'll see what I mean.
For example, the number for F bucket (upgradable class) from IAH to ANC, is often times different from the F number for SEA to ANC.
Look at this scenario, say from IAH, A1 F0, and from SEA, A1 F1.
In this case, a Silver from SEA might get upgraded to F, rather than a Gold from IAH just because there is availability in F from SEA to ANC.
Hope this explanation helps....