There's much more air outside than inside, so 15C colder inside does not mean that the entire city gets 15C hotter outside. And in a heat event, most people are inside, not outside. 1C hotter outside to make it livable for 99% of humans sounds fine. And this is only about cities, anything living outside cities will be fully unaffected.

For the people that have to work outside: air conditioning in the vehicle, frequent breaks in air conditioned areas, and I wonder if we could get proper air conditioned clothing at some point (currently vests with fans embedded are quite frequent in Japan, but that's the best there is as of today).

But I agree with the last paragraph. Air conditioning is the only countermeasure we have but in the end the fact remains that many cities will eventually become incompatible with human life in summer.