CX J class from HKG to SIN is definitely NOT worth the price. I've flown this sector in J on a 777, I've flown a lot of regional CX flights and long haul flights, and the J cabin is different on regional flights and long haul flights. Seats on CX's regional J cabins do not have the same features as those on the long haul flights. You need to check what type of equipment is being used for your flight, however be aware that CX is notorious for changing equipment right up to the last minute. There have been times when I've had a great seat assigned only to show up at the airport to find that the equipement has been changed and my great seat is no longer so great. I'd say, save your money, the $12,500 can be spent elsewhere on the trip where you'll get more value for money.

As for HKG - SIN/BKK - HKG, I can't help you with your questions, I do have a suggestion if you are not set on flying OW airlines you could consider SQ, HKG - SIN - BKK - SIN - HKG, with a stopover in SIN in one direction. If you are interested in flying on the A380. SQ has one SIN - HKG - SIN a day on the A380. Also, this will give you a flight between SIN and BKK, as CX does not operate these flights and your CX ticket would be an open jaw ticket from HKG - SIN/BKK - HKG.