My two penneth on this:
A. I thought further mutations would lead to milder infection, at least that was my interpretation of the science.
B. We have the vaccine and drugs to treat infection so why bother requiring a test to enter the country?
A: There is no way of predicting whether mutations would lead to milder or more severe infection - it's random mutation followed by selective pressure. In many cases a virus WILL mutate to produce a milder infection, because that is the best way of it passing from one host to the next - Omicron has essentially become fixed in the UK population because the vaccine, in combination with that old chestnut "herd immunity", means that (in general) people get a milder disease, don't drop down dead or have to isolate, and are able to pass it on. That is all a virus "wants" to happen.
However, the worry is that a new mutation will occur that leads to a much more serious illness and can potentially evade the vaccines. China's vaccine programme has been, according to all available data, shit - which is why the moment they have released people from their "zero covid" measures, about one-quarter of the entire population have become infected. They can then get infected with multiple variants at the same time which can lead to crossover of genetic information and potentially a new and quite different strain, even perhaps with a new coat protein.
Most of the vaccines are targeted at the Spike protein - if that mutates too far, it MAY evade the vaccine, but it is more likely that it simply won't bind to its natural receptor so won't cause disease.
Sorry for the complicated and slightly circular answer!