In terms of distance traveled, SEA-ATL-JNB, SEA-LHR-JNB, and SEA-CDG-JNB are all pretty equivalent. SEA-YVR-HKG-JNB is another ~2500 miles of flying each way.

Delta ATL-JNB in Business Elite in Low (sole award band for AS redemptions) is legendarily tough.

BA's taxes and surcharges as cited by BA Exec Club for SEA-LHR-JNB R/T in First are $1,371.29. Alaska may not demand that to the penny but it's going to be a lot of money.

Given AS' one-partner rule and subsequent need for award availability on the same carrier in both directions, unless the OP has extensive travel date flexibility I think the idea of airline choice is an illusion. (I've done CX in long-haul business and first and can recommend it without hesitation.)