Can i purchase single ticket for 72 transit visa?

Im traveling Stockholm - Hong Kong staying there 5 days and after that Bejing - Stockholm.

So i will have to buy a single ticket from Hong Kong to Bejing and was thinking of maybe go there 2 day before my travel back home, will i get 72hour free transit though my ticket is purchased in that way?

Techincally my route will be Hong Kong (country a) - Bejing (country b) - Stockholm (country c).

Do we need to tell the check in desk before boarding or can we do it in bejing airport? Is the officers scary to speak with or are they nice and let most people explore their awesome country?