[David]

Can you cite some sources for your responses to 1 and 4? Re: 5, there's clearly a distinction between typical mutations, which have no particular effect on the virus' behavior but do allow us to trace infections, and behavioral mutations, which might allow reinfection or make the virus more or less contagious or deadly. I think GP is asking about the second kind, since they're also asking about reinfection. Maybe that's obvious, but considering that, are there sources saying that the evidence so far is that there is only one strain?

Re 6: this is an interesting question. My layman's understanding is that heat usually reduces the spread of disease in temperate climates both because the disease can't survive as well and because people spend more time outdoors and in less people-dense spaces. In somewhere really hot like Qatar, we would expect to see the opposite of the second effect; I'd think people would spend more time indoors with A/C when it's excessively hot outside, increasing transmission rates as the outside temperature rises.