

In war, the economy does not slow down. It is turbocharged.
A nuclear war could counter global warming by triggering a nuclear winter but the actual effects are very uncertain. Basically, for a nuclear winter, a lot of “dust” needs to be lifted into the stratosphere. Those huge, multi-megaton bombs that they had back in the day caused a mushroom cloud that rose all the way to the stratosphere. Today, smaller, more precisely targeted bombs are preferred. It also depends on how combustible the targets are. No one is really quite sure what the climate effect of nuking a city is.
ETA: That was how climatologists saw nuclear winter ~15-20 years ago. No idea if anything has changed, but there probably wasn’t a lot of new data.
A substantial reduction in the human population would largely end the burning of fossil fuels and trigger reforestation; removing CO2 from the atmosphere. Ultimately, I would expect WW3 to greatly mitigate global warming.
Mostly, they can’t. Businesses like Walmart have margins of around 3-4% or so. If the cost goes up an extra 10% or more, that’s it.
Besides, I don’t see why they would prop up a guy who’s ruining them, anyway. Even if it didn’t require gifting billions to customers.