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.

In a dense residential community, when 70% of the units are running ACs and a minority are not, it's going to get substantially hotter than 1C for the ones that are not. It will be upward of 5C hotter in the non-AC units in my experience when the wind is minimal. The 1C you came up with does not apply in the close proximity of the dense urban air conditioning.

Reflective clothing using PDRC materials will be a lot more feasible than personal air conditioning. The latter would require a powered spacesuit anyway which makes it awkward to work. See https://youtu.be/NVAcSgLZues although it's not about clothing, but the idea is the same.