That first article (or the abstract at least) doesn't actually suggest this.

It shows people who have had heatstrokes are more susceptible to them. That's not the same as the previous heatstroke causing or contributing to their susceptibility.

Makes sense that people who were at higher risk already are more likely have had one in the past.

There's no explanation there. If we did find Neanderthal mitochondrial dna in some human population, that would just mean mitochondrial eve would have lived longer ago than we thought (before Neanderthal and sapian lineages split.)