If you're considering Korean using DL/CO points, a couple things to consider.

First, use DL rather than CO (in general) since AX offers more bonuses for transfering the miles over to DL. Both CO and DL transfers are pretty much instant.

Second, you won't get a KE F reward. You can only book in to their business class product using partner points.

Third, there are a lot of blackout dates in the KE program, though mostly in the summer, not the winter.

If you are set on your dates and are willing to pay double points to confirm the seats, going via CO will let you buy an EasyPass ticket on NW, while DL does not have that option. I actually just did a quick search and most dates between 12/15 and 1/10 have nothing, but the NW flight on 12/29 has at least one seat at a standard rate for SFO-NRT, and there is plenty of availability in mid-January for the return.