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"HKG-LAX on CX metal in J for 100k AAdvantage miles"
HKG-LAX in J for two costs 110K, not 100K AA miles. If you will not have redeemed any AA miles for air travel in the year you make this redemption, then you will receive a 10K rebate of miles, as long as you have a qualifying AA credit card open and in good standing. But initially, you will have to redeem 110K miles.
I think the biggest problem with your plan is your assumption that you will be able to find two premium-cabin seats on the same flights for your exact dates of travel. It's possible that you will, but it's far from certain.
I am not very familiar with SQ's program, but does SQ have routing rules in connection with its stopover awards? If so, do your planned itineraries comply with those rules?