72 hour visa free- HELP

I am flying from USA to Ulaanbaatar. I have confirmed flight and confirmed seats for flight from USA to Beijing(Overnight stay), from Beijing to Ulaanbaatar, and from Beijing back to USA. My return trip back AFTER my tour has a confirmed flight on Air China from ULB to Beijing, but NOT a confirmed SEAT yet. The confirmed seat from ULB to Beijing will be given to me during the tour. Will I have any trouble getting a 72 hour free transit? I will need 2 separate 72 hour free transit, correct? One once I reach Beijing from USA and one from ULB back to Beijing before I leave the next day to fly back to the US. The lady from Air China said I must have proof of 4 confirmed seats on ALL my flights (I only have 3 confirmed seats currently) once I reach Beijing from US in order to get a 72 hour free transit. Is having proof of a confirmed ticket enough?