That flight leaves Beijing about 7-ish in the morning. I'd feel comfortable advising you to not worry about anything as long as your intl arrival into Beijing is not earlier than 24 hours from that, on the previous day. Then you can use Transit-Without Visa privilege and of course, leave the airport during your layover in Beijing.
I'm not sure what the procedure is for this flight is currently, since it is an oddball and HLN is a real outstation, but for this one, they might want to process you through international immigration at Beijing, not Hailar and particularly since you have a through-ticket PEK-HTA. With this short of a stop in HLN, they may have Chita-bound passengers stay on board there, or they may segregate those passengers and put them in a holding pen while they do refueling then reboarding, or may corral all of you and have you do the official exit there. Probably you should just not worry too much and once you get into PEK and through the transit immigration, head for Air China desk and see if you can get clarity for the onward procedures. Then go with the flow.