laurin I certainly believe you, for what it's worth, a really ghastly situation you've been put in.
Paypal ought to monitor these claims a bit more slowly and consider the history of buyer:seller but they don't. I know they favour the buyer and as an individual buying on e-bay from larger commercial sellers this is good. I live in the EU and would not like to have no protection when buying from abroad the US/Asia. But in this situation, it is inter-EU and obviously private individuals not commercial bodies. In such cases, I think Paypal should be more cautious.
Ideally, packaging and weighing and handing over of packets to postal workers should be filmed, but practically in busy post-offices this is impractical. What MIGHT be a good idea is for sellers to send pictures/video immediately to the buyer which could deter any scamming intentions.
Paypal should pay attention to the weight anomaly on your sealed package, the buyer's returning a lighter package is significant. I very much understand your attitude to phone calls, these are lengthy, international expensive and likely you get somebody who just is told to parrot corporate answers back to you. I feel Paypal should have a free Skype type of link if they really want to protect both parties interests. I'm rather sure your French scammer has a nice little history of these kind of actions.
It makes you wonder how anything gets done on eBay/paypal, but it does for the main. In 9 years I've only ever had one seller try to rip me off with non delivery&flaking, Paypal fixed it for me. But, I do understand your distress and bewilderment, you've been stolen from.:grumpy:
Regards, Will