Yes, you can check bags from BKK to SFO. However, when you arrive in HKG, I strongly suggest you go to the United transit desk and make sure they have the bag tag info. My wife did this last year without any problems.

I believe the HX desk doesn't open until 2 hours before the first departure, so around 1:20am in this case.

As an aside, it's likely you will only be given the BKK-HKG boarding pass by HX. Once you land in HKG, you can then check in online or do so at the transit counter.

No same-day confirmed applies for HX flights on a UA ticket, regardless of UA status.

For more info, you may wish to review this thread:
https://www.flyertalk.com/forum/unit...ua-flyers.html