I think assessment will be best done by a respectable art or antiques dealer in your region. I have been here for a number of years, and none of the veteran members here are experts in the area.
Having said that, my personal (amateur) opinion is that there is no telling whether it is Japanese or Chinese in origin.
The dragon is closely linked with water (the Japanese "suijin," the water god, is a dragon), but this belief originates in China.
Also, the ornateness of the design is very rare in Japan, considering the Japanese preference for austerity and simplicity in design. So, my first impression was that it is Chinese in origin or inspiration.
Is it a teapot or a sake pot? I don't know that either. But boiling in a silver pot is believed to "soften" the water--an idea shared by Chinese/Japanese and British tea service styles. On the other hand, the size of the pot is appropriate for warming sake, too.
Conclusion: I have no idea...
BUT, I can say that, if I were you, I won't be boiling water in it. I have a feeling that it is worth a lot of money. I'd keep it way from dust and corrosion in a case or protected shelf.