According to Alaskaair.com's Web Check-In Page, "Customers traveling to an international destination with Alaska Airlines or Horizon Air are also able to use Web Check-In., so not sure what the problem might be here.

Regarding your possible membership in Northwest's World Club, I strongly recommend you check out Priority Pass. Not only will your $399.00 one year membership get you unlimited access to Northwest World Clubs, but you'll also have access to AS, CO, DL, HP, UA and US lounges regardless of whether you're flying that day with the specific airline or not. Only downside is that you'll have to pay for any guests at $24.00 per guest. On a more positive note, you'll have access to huundreds of overseas lounges, including Aeromexico's Salon Premier at Cancun.

Good luck!