It's quite unlikely that you would be denied boarding on your flight back to the US.

Essentially you would be running the risk of being denied future entry to China, either on a similar transit visa or a standard one, for having violated the terms of your visa. (ask anybody who's overstayed a visa in the US how that worked out for them!) It's up to you to decide whether you want to run that risk.

If you're so unwell that you don't want to fly to HKG, I'm not sure that flying overnight 10+ hours to the US is a better idea...

Whatever option you choose, I hope you feel better.