The imperial attacks weakened the Chinese military and infrastructure considerably... how do you enforce a treaty that is inherently unenforcable? But that was the whole point you see. The imperialist powers knew it was an unbalanced treaty, and the lack of its enforcement gave them entitlements for further invasions and further unfair treaties. This is the same tactic they used in every nation they invaded. They claimed that the occupied government was not doing its job, then continued invading until they were essentially given full control of the country.
You can't use the claim that the Qianlong Emperor didn't honour his agreements and therefore China deserved to be looted. The Treaty of Nanjing and subsequent treaties were unbalanced and unenforceable in the first place.
I personally do not souly blame the imperial powers for what happened in China. The Emperor and his xenophobia also contributed to the events which destroyed most of the ancient mainland... but as you know, Chinese history is incredibly complex and a domino effect, and so you cannot blame it all on one factor.
Court rooms don't operate on what "could have" happened. Your parameters were unrealistic in the first place.
I expect the French government, and all nations with stolen property, to do what is right and return property within its borders to other nations who are demanding them back. No demand? No problem. France and Britain have tonnes of outstanding claims from foreign countries that want their heritage back. France itself has claims against Cuba, parts of Africa, etc. for return of its priceless and irreplaceable art. Yet France itself will not acknowledge claims against it for stolen property? Utter hypocrisy.
If relics which are now in private hands were originally looted by any establishment in relation to the French government, then yes, I expect them to be confiscated and returned. What makes those relics safer in a private collection than in a museum where curation is paramount, and where the public can have access to view their own heritage?