1) Letting the Yuan rise while the US is intent on fucking over China's trade engine is the same as the Plaza Accord which punted Japan's economy into their lost decade when the Yen appreciated,
What led to Japan's lost decades - Plaza Accord in 1985, or Busting of a massive property bubble in 1991-1992, or slow deceleration due to demographic issues beginning in 1960s and exacerbating in 1990s, or competition from other countries/regions (China, Korea, Taiwan, Europe), or missing the Digital bus completely - is a major policy debate, that can't just be dismissed as due to plaza accord.
2) Gaining Forex (not explicitly accumulating USD since Chinese trade with the US is literally collapsing -- down 30%) is not even the main goal. It is stabilizing industrial output and employment when the US is actively trying to fuck you over.
Industrial output can also be stabilized at 6.3 USDRMB exchange rate. It's just 10% increase which will not have much impact on Chinese exports, but lead to greater usage and holding of RMB (everyone likes to hold an appreciating asset).
When China is facing a historic unwinding of a historic RE bubble while the US and West is throwing up tariff after ban after embargo, you do not do what the Japs did and make your products more uncompetive by helping the US with a secondary tariff which is what a currency rise would do.
This logic has no endline, you can go to forever depreciation. What the optimum exchange rate is a complex business, but current exchange rate is highly depreciated. Just a 10% appreciation, which will be say 5-6% real imported input adjusted appreciation, is nothing that Chinese exporters can't handle.
In fact the people who can't handle that
must go bust. It is a requirement of an economy to have creative destruction so that people keep rising up the value chain gradually. China right now has a labor shortage in the factory sector.
(BTW, there is deflation in China the spending power for Chinese goods is greater than ever. A currency rise would only give Chinese more spending power for foreign goods which is exactly how you fuck yourself over in trade war and undermine your own industries.)
Precisely, it would give Chinese people more spending power, but they won't spend that on foreign goods which are not competitive but on domestic goods. If the RMB appreciates by 10%, leading to say reduction in commodity costs for cars, making cars cheaper still, Chinese people are not going to buy imported cars, just more expensive domestic cars. That leads to demand increase.
USDs are a declining part of China trade returns. And they are no longer automatically converted back into US intruments like treasuries. They are being converted into gold, copper and assets in the Global South. Removing the risk of USDs can't happen overnight but it is happening.
They are not kept in the forex reserves, but they are still USD kept by state banks. All gold, copper purchases are already part of import data. If you are talking about assets in Global South, US can do one military operation to seize any such asset in most of the global south, or the global south countries themselves from time to time seize assets when governments change (common in Africa).