[Loli-Knight]

So is getting a refund for empty box/wrong item/etc  from Amazon Japan (as someone in the Us) if I buy one of these kits the same as when I do it with US Amazon?

No difference - you're still dealing with the company, no matter where you reside and have the package delivered.

In terms of the empty box/missing Item method, what's the precise weight of the model kit?

1.18 KG according to the Amazon page, which seems to be about average for this particular line of kits these days.