72 hour transit without visa query

Hi,

I will be travelling from London to Bangkok next February with my partner and our stopover is in Beijing for 12 hours. As we will arrive at midday it would be quite nice to wonder around that day - what will I need to do? I understand I can get a stamp in my passport for 24 without a visa.

However my second query is that after we have been in Thailand for two weeks, we will be flying back to Beijing for just under 72 hours before returning to London. Collectively over the period of travelling back and forth we would have been to Beijing for just over 72 hours. Does this matter? Or would they stamp the passport again?

On my return, I will land in Beijing at 10pm and then will be going back to London on day 3 of my stay but at 2pm in the afternoon.
Is it still correct that my 72 count day will start at 00:00 following my day of arrival?

I am nervous about going with no visa documents but at the same time don't want to get it if I don't have to.

Regards