Here's how to decipher the fare basis. First letter is the booking class. Second and third letter is usually the season and day (L & H are low and high season respectively, W & X usually mean weekend and other days respectively). The last few alphanumerics are usually optional to indicate the timeframe restriction (3M = 90 days before returning, 6M = 180 days).

Why not route LAX-NRT-SIN on NW instead? CO codeshares with NW on these routes. DL's routing and search engine going to SIN involves CI (connecting in TPE) or AF (connecting in CDG). Easiest way to check is to download the PDF of their timetable. Hence, the KE flights into SIN will not be code-shared with DL. Keep in mind you're often allowed a stopover with higher fares (HNL, PDX, SEA, NRT) through the Pacific. You'll have to ask around about the transAtlantic fares.