what if 72 hour visa entry is not accepted?

Travelling from Canada to Beijing - 2 day stay-over, then over to Thailand for 10 days. Leaving Thailand, transferring back through Beijing to Canada. Arrive in Beijing 17:30 on 22th, leave Beijing for Thailand 13:50 March 25th (68/69 hours) - return to Beijing on April 5 (transfer only, no airport exit) for 8 hour lay-over then flight back to Canada. Based on the hours of arrival/stay, I am within 72 hour limit - but what if for whatever reason I am refused? As return home is transfer only, I am not counting those hours towards my 72 limit. Just don't want to get all the way over there and be denied.