no lounge or J class is worth the trouble you are going to go through.

in your case book on 2 seperate hkg sgn flight on same day.

so your Y leg of ICN HKG SGN book on 10pm CX719 connection, and issue ur business HKG SGN one way on 4pm cx 799 assuming you land to hk earlier than that... CX IT is smart enough to spot double booking on SAME flight but hopeless beyond that.