Flights on Alaska partners do accrue at 100% for the partners listed. However, you need more of them to make the elite levels. For example, it's 25K for MVP, and then with partner fights it's 30k.

When you look at your account on-line it breaks it down for you by showing you your next milestones by Alaska Flight miles needed, and then by partner miles needed.

For example:

YTD Alaska/Horizon Miles
Miles Flown: XX,514 XX,486 Miles to MVP Gold

YTD Alaska/Horizon/Qualifying Partner Miles
Miles Flown: XX,473 XX,527 Miles to MVP Gold

YTD Alaska/Horizon/Qualifying Partner* Segments
Segments Flown: XX XX Segments to MVP Gold

As for COntinental, you are correct getting here from other places is fine, but the only flights out of SEA seem to be to their hubs Houston or Newark. If you want to check, then ask for a flight between SEA and SFO or ORD. All the choices come back going either through Newark or Houston. Great for running up the mileage tab, but I don't think my boss'll like that very much. Honest, I;d love to stick with them,. but that's a bit impractical

So I have done some homework, and really aren't that biased.