Never flew the above airlines, so can't comment on the C service.

Asiana (OZ) will credit full miles to SQ.

General procedure for inbound US flights is that you have to clear customs at the first port of arrival to the US. So you would normally check/tag your luggage all the way from BKK to LAS, but receive it in SFO/LAX go thru customs there. Usually the airlines have luggage belts to drop your stuff just after customs.