My understanding is that in the long-run, you cannot be the world’s reserve currency AND hold a trade surplus. It is called the Triffin Dilemma.
Oh, and they have to let it freely float, obviously.
Only in downturns as an "emergency response", but that's when people want to short sell them and when they are most likely to be overpriced.
> The answer, it turns out, is a five-point plan that is conceptually straightforward, historically grounded, and politically impossible.
The answer, based on western economic theory. There are other options. An illustrative example is that a military empire can mandate tribute paid in their currency, essentially forcing global adoption. China hasn't shown more of an inclination to economic incentive though. Tie belt-and-roads development to economic integration and RMB will be used, even if there are huge flaws as shown by the economic theory.