I think that the issue is one PNR or two from REP to JFK.
If on one, you'll be protected by KE in case of irreg ops, and I honestly can't see any benefit whatsoever in introducing an additional stop (BKK) and additional airline (Bangkok Airways). If on two, then I'd concur with your TA and advise planning on an additional day in Seoul.
[My wife and I flew to/from Siem Reap on SilkAir (SQ subsidiary) from SIN, and it could not have been more dependable.]