Is it possible that China could manipulate it's currency so Trade war doesn't mean anything at all?

The Short answer to this is "Yes" or "maybe". However, the real question is:" Will the rest of the world put up with this?"

If China can do this likely it would, which could or would harm not only U.S. business interests but business interests of all countries worldwide. In other words if China manipulates the value of their currency downward beyond a certain point no country on earth could economically compete with them.

In some ways doing this would not harm the average Chinese person in China either because of the size of the country which is why it is likely that China would try this to harm the U.S. and Europe and other countries that might be supporting the U.S. in the Trade War.

It would be an attempt to prevent the sale of any American goods overseas anywhere by undercutting it by pricing All Chinese Goods under any U.S. prices anywhere on earth.