Multiple 72-hour visas and day trip to HK?

I'm a Canadian citizen. Is this doable:

Canada > Guangzhou (2 days) > Laos > Burma (I have visa already) > Guangzhou (2 days) > Canada.

First question: can I apply for the 72-hr free transit twice? I enter Guangzhou twice.

Second question: I won't have my Laos visa on hand as you apply for it at the airport there - is that okay?

Third question: when in Guangzhou can I take a day trip to Hongkong and then get back into Guangzhou with my 72-hr free transit?

Thanks!
Pearl