How do you suggest we would be able to completely engineer, from scratch, a novel virus with 1200 mutations spaced randomly throughout the genome, that would rescue properly and be capable of infecting a novel host? I know of no such modeling technology to accomplish this. All of these are phenomena that happen easily and constantly in a massive natural reservoir of any viral population.
And if you think it was not strictly "engineered" but instead "evolved" selectively by hand, then how can you escape the S/NS ratio problem? How can you escape the adaptation and glycosylation problem I describe in my original post? How could you avoid the issue of adapting to the host it was evolved in, and acquiring these "lab animal" or "cell culture" specific adaptation mutants that would then need to be somehow removed. Or it likely would not infect humans with any great reproductive efficiency. And then it would have an S protein that would be a bit more adapted to humans, as it has become over the course of the last year. As it's evolving in us, it is becoming better at infecting us. If it had been evolved in humans prior to the pandemic, this would not be as necessary and it would not happen on the scale it's currently happening.
And when exactly would China have started this little "experiment?" Where? With what subjects? How did they keep it secret?