I hold a German passport and want to attend a conference in Guangzhou for 2 days.

I have a confirmed flight from Seattle to Beijing to Guangzhou... leaving from Guangzhou via Shanghai to Seattle within 72 hours by any measure. However, I am not sure if I meet the criteria for entry airport must be same as departure? Do I need to apply for visa in Beijing or can I stay in connecting flight area and apply for entry permit in Guangzhou, from where I also depart. In that case I assume I would meet the criteria.

The other concern is that I am not connecting to a 3rd country. How strict is that...? Assuming the above is not a not a problem, could I meet the 72 hour free transit requirement?
Seattle to Beijing to Guangzhou
Guangzhou to Hong Kong to Seattle

Or maybe an alternative would be to fly from China to Vancouver , Canada on the way back, rather than fly to Seattle, USA.

I do have an invitation letter, but to get an entry permit I would need to make a trip to San Francisco from Seattle, which I am trying to avoid.
Alex