Beijing- KL, KL-Beijing, Beijing-Prague

Hi,

Please could you just confirm if I am right. I am so confused...
Situation: I am about to fly from Beijing for holiday to Malaysia( Loosing my one Entry student visa). There I spend 2 weeks in Malaysia and fly back to Beijing(wanna use 72-hours-free-transit) pack my things at the appartment and fly to my home country(Prague, Czech republic). I think I meet all requirements to apply for 72hour-free-transit.

So...
1) Where can I apply for 72-free-transit? On my way from Beijing to KL? On my way from KL to Beijing?
2) I can read that at Beijing Airport they do not do it in advance. So is it safe to buy flight tickets and than just hope everything will work?
3) I have already bought tickets from BJ-Prague so I hope its ok I will have tickets from different airlines, right?

Thank you very much for confirmation! YOu content is amazing, using it everyday on my trips in CHina:-)
Regards,
Karolína