"It's very singularly focused on the spike protein, so if the virus evolves such that the spike protein becomes less important, all the vaccines so far will then become useless to that variant."
COVID-19 is entirely covered with its spike proteins - hence the term Coronavirus. What else would you suggest that we prime the immune system to recognize?
"Vaccinating during an active pandemic puts evolutionary pressure on coronavirus which makes this more likely to happen. Vaccinating some of the world population while huge amounts of coronavirus is still circulating is therefore risky."
It's quite the opposite. The more hosts the virus has to infect, the quicker it will mutate and render previously acquired immunity less effective. If we could vaccinate everyone against the current strains at once, herd immunity would be reached despite imperfect vaccines and the pandemic would be over. Since that is not possible, we should vaccinate the maximum number of people possible, in as quick a time frame as we can.
"there could be an argument to be made that, a bit like chicken pox, it's better for children to be exposed to covid when they're young and won't be seriously affected, but should then have antibodies for future protection."
Childhood illnesses like chicken pox and measles confer lifelong immunity because their viruses do not mutate rapidly. COVID-19 , on the other hand, does mutate albeit not as rapidly as influenza viruses. Given its track record in the last two years, it is highly unlikely that natural immunity to COVID-19 acquired following infection will protect you for life. Therefore, your idea that children should be infected now so that they are protected as adults does not hold water.
Having said that, we are offered Pfizer vaccines for +12s here but our DC are not yet vaccinated. DH and I are fully vaccinated but do not feel that the risk/benefit ratio for teens is as low as we would like just yet, given the unknowns and lack of data. Such is the shortcoming of voluntary vaccination programs, with which it has been shown that no disease can be eradicated.