Who knows, sunlight and seasonal immune settings seem to tamp it down…but could this selection pressure on the virus be what then creates the severity of pandemic 2nd waves?

Historically, when viruses mutate, they most often mutate to less virulent forms. I don’t think there’s any reason why they cannot mutate to a more virulent form, especially if we’re putting evolutionary pressure on them to do so, but that’s generally not the pattern seen in vivo.

If so we need to get ahead of spread of any more lethal strains by insistence on disinfection of public transport etc and maybe preventative lockdown going into autumn (fall) would knock the wind out of it further.

Yep. The NYC infection pattern indicates that the most efficient mode of virus transmission are rapid transit systems like trains and buses.

Also someone needs to work out how to sanitise air-con and ventilation for workplaces.

This is doable, actually. A HEPA based system that includes ultraviolet irradiation is going to come pretty close to 100% when it comes to filtering out virus particles. Somebody just has to work out the details for each building ventilation system. Not particularly difficult to do.