In general, I think that earlier closures would potentially have delayed spread enough to save lives due to getting vaccines and testing further along than they were.
I’m also claiming that now, with a fully in place and adequate test-and-trace program, including screening for passengers and isolation for positives, border closures have low marginal benefit. Without such a test and trace program, travel modifies the spread dynamics by little enough that it won’t matter for places that don’t have spread essentially controlled. The key case where it would matter is if the border closures delayed spread by long enough to put in place such systems, in which case they would have been very valuable. And yes, border closures in place have allowed this in some places, but certainly not the US or UK.
So, conditional on the policy failures, I think border closures were effectively only a way to signal, and if they distracted from putting in place testing and other systems by even a small amount, they were net negative.
I don’t understand the hypothetical.
If every country in the world had closed their borders well enough to stop all movement before it left China, yes, spread would have been prevented. But that’s unfeasible even if there was political will, since border closures are never complete, and there was already spread outside of China by mid-January.
Once there is spread somewhere, you can’t reopen borders. And even if you keep them closed, no border closure is 100% effective—unless you have magical borders, spread will inevitably end up in your country. And at that point, countries are either ready to suppress domestic spread without closures, or they aren’t, and end up closing later instead of earlier.