I do not know for LAX, but it is likely that checking is handled by a third party company. Their agents might not be well trained on how to handle your case. I would try to get your two BP issued at LAX by the same agent (rather than OLCI separately). I would try to locate a CX supervisor to handle your case.
Of course, you should try to get BA to link your two PNRs, but I have a scientific doubt that it will achieve much.
Also remember that their are frequent ac swaps on HKG-SIN, so you might be downgraded to J on that route.