Does 72 hour visa free transit apply? Us to Guangzhou

Yes, you are perfectly qualified for 72 hours visa free transit in Guangzhou. Also, you have correctly mentioned that you will have to fly from Guangzhou to Hong Kong in order to enjoy this convenience. By the way, you don't have to worry about Guangzhou to Hong Kong flight because Hong Kong is considered as third country within the rules regarding 72 hours visa free transit.

You layover in Japan can be just a few hours. This is not important. The only thing that is important is the fact that you are arriving to Guangzhou from Japan by nonstop flight. Anyway, you can have US as your origin country, as far as you use Hong Kong after Guangzhou. Hypothetically, if you skip Japan you would have:

US - Guangzhou (mainland China) - Hong Kong. This is eligible without any doubt.

As far as the tickets are concerned, you really don't have to worry. The only thing that is important is your way (direction) of travel. So, just be relaxed and choose whether US or Japan will be the country from where you will arrive in Guangzhou.

Keep in mind to have your documentation, i.e. flight tickets and accommodation bookings in printed form upon your landing in Guangzhou. Also, you should know that 72 hours start the day following your arrival at 00:01 and end three days later at 23:59. So, if you want to enjoy your time in the best possible way, the best option would be arrival in the morning hours and departure as later as possible.