As per the rules given in TWOV and the new rules for TWOV for Guangdong, I am assuming YYZ -> CAN -> LAX, is compliant. Do the Chinese have a sort of "unwritten rule" about transit being out of the way?

I am booking a separate one way ticket for ATL to YYZ/YVR. Then a multi-trip ticket for YYZ/YVR to CAN then CAN to LAX to ATL.

Second, since HK is a third country, would I also be allowed to go from USA to Guangzhou, then Guangzhou to HK by train, then HK back to USA?